Address Doc + Format CI failures and CodeRabbit review feedback on
PR #439:

- Doc: clear ambiguous fn-vs-module intra-doc links in `lib.rs`,
  replace links to private items (`satisfies_with_prerelease`,
  `dedupe_current`) with plain references, and remove the
  cross-crate link from `graph-hasher` to
  `pacquet_package_is_installable`.
- Format: re-flow `installability.rs` after the previous edit
  broke the `node_version` line shape `cargo fmt` expects.
- `InstallabilityError`: add an `InvalidNodeVersion(InvalidNodeVersionError)`
  variant so callers keep the upstream `ERR_PNPM_INVALID_NODE_VERSION`
  error code instead of seeing it collapsed into a synthetic engine
  mismatch.
- `InstallabilityHost`: add a `node_detected: bool` flag that
  `install_frozen_lockfile` consults when deriving `engine_name`.
  Restores the prior "None when no `node` on PATH" semantics so a
  detection failure can't poison the side-effects cache with rows
  keyed on the `99999.0.0` synthetic fallback.
- `InstallabilityOptions`: switch the string fields to `&'a str` and
  derive `Copy` so `compute_skipped_snapshots` can build the
  host-derived part once and toggle only `optional` per snapshot.
  Removes four `String` allocations per snapshot.
- `satisfies_with_prerelease` doc comment now lists both known
  divergences from upstream's `includePrerelease: true` (the
  `>=X.Y.Z` over-acceptance the ported test pins, plus the
  `<X.Y.Z` under-acceptance), not just one.
- Move the prerelease known-failure case from `#[ignore]` to
  `tests/known_failures.rs` so `just known-failures` picks it up
  alongside the existing lifecycle-script entries.
- Dedupe the doubled "Resolve the host context from the running
  process." doc summary on `InstallabilityHost::detect` after the
  earlier rustfmt run.

…ed (#434 slice 1)

The cold-install CI benchmark on PR #439 showed a ~3% regression
(2.075 ± 0.072 s vs 2.009 ± 0.055 s on a 1352-package fixture).
The dominant per-snapshot cost in the new
`compute_skipped_snapshots` pass is the `snapshot_key.without_peer()`
+ `metadata_key.to_string()` + `package_is_installable` chain.
Skipping that entirely whenever the lockfile contains no metadata
row with an `engines` / `cpu` / `os` / `libc` constraint avoids
the per-snapshot work for the common case (most real lockfiles
have only a handful of platform-tagged native packages, and many
have none).

`any_installability_constraint` is a linear scan over `packages`
that short-circuits on the first declared constraint, so the
fast-path probe is O(1) amortized on lockfiles that do have
constraints (it finds one quickly) and O(N) on those that don't
— but with a much smaller per-row cost than the loop it replaces.

Also drops the redundant `or_else(|| Some(vec!["any".to_string()]))`
allocations in `check_package`. Upstream's `index.ts:82-86` defaults
absent platform axes to `["any"]` before passing them to
`checkPlatform`, but pacquet's `check_platform` already skips an
axis when its wanted field is `None`, so the default was
allocating one Vec + String per axis per snapshot for nothing.

Adds a unit test that exercises the fast path against a synthetic
constraint-free lockfile.

Address Doc + Format CI failures and CodeRabbit review feedback on
PR #439:

- Doc: clear ambiguous fn-vs-module intra-doc links in `lib.rs`,
  replace links to private items (`satisfies_with_prerelease`,
  `dedupe_current`) with plain references, and remove the
  cross-crate link from `graph-hasher` to
  `pacquet_package_is_installable`.
- Format: re-flow `installability.rs` after the previous edit
  broke the `node_version` line shape `cargo fmt` expects.
- `InstallabilityError`: add an `InvalidNodeVersion(InvalidNodeVersionError)`
  variant so callers keep the upstream `ERR_PNPM_INVALID_NODE_VERSION`
  error code instead of seeing it collapsed into a synthetic engine
  mismatch.
- `InstallabilityHost`: add a `node_detected: bool` flag that
  `install_frozen_lockfile` consults when deriving `engine_name`.
  Restores the prior "None when no `node` on PATH" semantics so a
  detection failure can't poison the side-effects cache with rows
  keyed on the `99999.0.0` synthetic fallback.
- `InstallabilityOptions`: switch the string fields to `&'a str` and
  derive `Copy` so `compute_skipped_snapshots` can build the
  host-derived part once and toggle only `optional` per snapshot.
  Removes four `String` allocations per snapshot.
- `satisfies_with_prerelease` doc comment now lists both known
  divergences from upstream's `includePrerelease: true` (the
  `>=X.Y.Z` over-acceptance the ported test pins, plus the
  `<X.Y.Z` under-acceptance), not just one.
- Move the prerelease known-failure case from `#[ignore]` to
  `tests/known_failures.rs` so `just known-failures` picks it up
  alongside the existing lifecycle-script entries.
- Dedupe the doubled "Resolve the host context from the running
  process." doc summary on `InstallabilityHost::detect` after the
  earlier rustfmt run.

…ed (#434 slice 1)

The cold-install CI benchmark on PR #439 showed a ~3% regression
(2.075 ± 0.072 s vs 2.009 ± 0.055 s on a 1352-package fixture).
The dominant per-snapshot cost in the new
`compute_skipped_snapshots` pass is the `snapshot_key.without_peer()`
+ `metadata_key.to_string()` + `package_is_installable` chain.
Skipping that entirely whenever the lockfile contains no metadata
row with an `engines` / `cpu` / `os` / `libc` constraint avoids
the per-snapshot work for the common case (most real lockfiles
have only a handful of platform-tagged native packages, and many
have none).

`any_installability_constraint` is a linear scan over `packages`
that short-circuits on the first declared constraint, so the
fast-path probe is O(1) amortized on lockfiles that do have
constraints (it finds one quickly) and O(N) on those that don't
— but with a much smaller per-row cost than the loop it replaces.

Also drops the redundant `or_else(|| Some(vec!["any".to_string()]))`
allocations in `check_package`. Upstream's `index.ts:82-86` defaults
absent platform axes to `["any"]` before passing them to
`checkPlatform`, but pacquet's `check_platform` already skips an
axis when its wanted field is `None`, so the default was
allocating one Vec + String per axis per snapshot for nothing.

Adds a unit test that exercises the fast path against a synthetic
constraint-free lockfile.

Address Doc + Format CI failures and CodeRabbit review feedback on
PR #439:

- Doc: clear ambiguous fn-vs-module intra-doc links in `lib.rs`,
  replace links to private items (`satisfies_with_prerelease`,
  `dedupe_current`) with plain references, and remove the
  cross-crate link from `graph-hasher` to
  `pacquet_package_is_installable`.
- Format: re-flow `installability.rs` after the previous edit
  broke the `node_version` line shape `cargo fmt` expects.
- `InstallabilityError`: add an `InvalidNodeVersion(InvalidNodeVersionError)`
  variant so callers keep the upstream `ERR_PNPM_INVALID_NODE_VERSION`
  error code instead of seeing it collapsed into a synthetic engine
  mismatch.
- `InstallabilityHost`: add a `node_detected: bool` flag that
  `install_frozen_lockfile` consults when deriving `engine_name`.
  Restores the prior "None when no `node` on PATH" semantics so a
  detection failure can't poison the side-effects cache with rows
  keyed on the `99999.0.0` synthetic fallback.
- `InstallabilityOptions`: switch the string fields to `&'a str` and
  derive `Copy` so `compute_skipped_snapshots` can build the
  host-derived part once and toggle only `optional` per snapshot.
  Removes four `String` allocations per snapshot.
- `satisfies_with_prerelease` doc comment now lists both known
  divergences from upstream's `includePrerelease: true` (the
  `>=X.Y.Z` over-acceptance the ported test pins, plus the
  `<X.Y.Z` under-acceptance), not just one.
- Move the prerelease known-failure case from `#[ignore]` to
  `tests/known_failures.rs` so `just known-failures` picks it up
  alongside the existing lifecycle-script entries.
- Dedupe the doubled "Resolve the host context from the running
  process." doc summary on `InstallabilityHost::detect` after the
  earlier rustfmt run.

…ed (#434 slice 1)

The cold-install CI benchmark on PR #439 showed a ~3% regression
(2.075 ± 0.072 s vs 2.009 ± 0.055 s on a 1352-package fixture).
The dominant per-snapshot cost in the new
`compute_skipped_snapshots` pass is the `snapshot_key.without_peer()`
+ `metadata_key.to_string()` + `package_is_installable` chain.
Skipping that entirely whenever the lockfile contains no metadata
row with an `engines` / `cpu` / `os` / `libc` constraint avoids
the per-snapshot work for the common case (most real lockfiles
have only a handful of platform-tagged native packages, and many
have none).

`any_installability_constraint` is a linear scan over `packages`
that short-circuits on the first declared constraint, so the
fast-path probe is O(1) amortized on lockfiles that do have
constraints (it finds one quickly) and O(N) on those that don't
— but with a much smaller per-row cost than the loop it replaces.

Also drops the redundant `or_else(|| Some(vec!["any".to_string()]))`
allocations in `check_package`. Upstream's `index.ts:82-86` defaults
absent platform axes to `["any"]` before passing them to
`checkPlatform`, but pacquet's `check_platform` already skips an
axis when its wanted field is `None`, so the default was
allocating one Vec + String per axis per snapshot for nothing.

Adds a unit test that exercises the fast path against a synthetic
constraint-free lockfile.

…l tests

After rebasing on top of #439 (`pacquet-package-is-installable` + the
`SkippedSnapshots` set), the new test sites this PR added —
`per_importer_prefix_in_pnpm_root_events`,
`unsafe_importer_keys_error_before_filesystem_writes`,
`cross_importer_link_dep_symlinks_to_sibling_rootdir`, and
`custom_modules_dir_propagates_to_each_importer` — needed the new
`skipped` field on their `SymlinkDirectDependencies` builders. Pass
`&SkippedSnapshots::default()` everywhere; the per-importer install
behaviour these tests cover is orthogonal to the installability
filter, so an empty skipped set keeps the assertion subject the
same.

---
Written by an agent (Claude Code, claude-opus-4-7).

…l tests

After rebasing on top of #439 (`pacquet-package-is-installable` + the
`SkippedSnapshots` set), the new test sites this PR added —
`per_importer_prefix_in_pnpm_root_events`,
`unsafe_importer_keys_error_before_filesystem_writes`,
`cross_importer_link_dep_symlinks_to_sibling_rootdir`, and
`custom_modules_dir_propagates_to_each_importer` — needed the new
`skipped` field on their `SymlinkDirectDependencies` builders. Pass
`&SkippedSnapshots::default()` everywhere; the per-importer install
behaviour these tests cover is orthogonal to the installability
filter, so an empty skipped set keeps the assertion subject the
same.

---
Written by an agent (Claude Code, claude-opus-4-7).

…l tests

After rebasing on top of #439 (`pacquet-package-is-installable` + the
`SkippedSnapshots` set), the new test sites this PR added —
`per_importer_prefix_in_pnpm_root_events`,
`unsafe_importer_keys_error_before_filesystem_writes`,
`cross_importer_link_dep_symlinks_to_sibling_rootdir`, and
`custom_modules_dir_propagates_to_each_importer` — needed the new
`skipped` field on their `SymlinkDirectDependencies` builders. Pass
`&SkippedSnapshots::default()` everywhere; the per-importer install
behaviour these tests cover is orthogonal to the installability
filter, so an empty skipped set keeps the assertion subject the
same.

---
Written by an agent (Claude Code, claude-opus-4-7).

… fixture

`crates/package-manager/src/installability/tests.rs` initialises a
`TarballResolution` without the `path` field that #451 added (git-
hosted tarball sub-directory selection). The omission landed when
#439 (installability check, #434 slice 1) merged just before #451 —
neither PR touched the other's file, so the bug shipped on
`origin/main`.

CI was failing with:

```
error[E0063]: missing field `path` in initializer of `TarballResolution`
  --> crates/package-manager/src/installability/tests.rs:56:49
```

Fix is one line: `path: None`. The installability check ignores the
resolution shape entirely (see the comment a few lines above the
init site), so `None` is correct.

Origin/main grew a `path: Option<String>` field on
`TarballResolution` (PR #451, `feat(git-fetcher): install
git-hosted tarballs via preparePackage + packlist`). That PR
landed after #439 (`feat(package-is-installable): platform +
engine check`) and missed updating the `TarballResolution`
literal in `crates/package-manager/src/installability/tests.rs`,
so any PR opened against current main inherits a build break in
the `package-is-installable` test build (#445 hit the same
failure). My own `peer_dependency_in_lockfile_surfaces_unsupported`
test in `crates/real-hoist/src/tests.rs` had the same gap.

Add `path: None` to both literals so the test builds compile
against the post-#451 `TarballResolution` shape.

Includes the rebase of feat/438-slice-3b onto current origin/main.

… fixture (#455)

Landrace between #439 (installability tests fixture) and #451 (added
`path: Option<String>` to `TarballResolution`). Both merged green
against their own bases but main's tip won't compile:

    error[E0063]: missing field `path` in initializer of `TarballResolution`
      --> crates/package-manager/src/installability/tests.rs:56:49

Add the missing field with `path: None` so the struct literal matches
the current `TarballResolution` shape.

…bc detection (#434 slice 2)

Slice 1 (#439) wired `pacquet-package-is-installable` into the install
pipeline but the `SupportedArchitectures` input was always `None`, so
every install behaved as if the user had opted into "host triple only".
This slice closes that gap by wiring the three upstream input sources
pinned at pnpm/pnpm@94240bc046:

- `Config.supported_architectures` deserialized from
  `pnpm-workspace.yaml`, mirroring
  `getOptionsFromRootManifest.ts`.
- `--cpu` / `--os` / `--libc` flags on `install` / `add`,
  multi-valued, comma-separated. Per-axis CLI override replaces the yaml
  axis wholesale, matching `overrideSupportedArchitecturesWithCLI.ts`.
- Real Linux libc detection — replaces the slice 1 `"unknown"` stub
  with a `/lib*/ld-musl-*` probe cached via `OnceLock`. Mirrors
  `detect-libc.familySync()` semantics: `"musl"` / `"glibc"` on
  Linux, `"unknown"` everywhere else. No new dep — open-coded
  `read_dir` is cheaper than spawning `ldd --version` and works in
  slim containers without `ldd` on PATH.

Threaded through `Install` / `InstallFrozenLockfile` / `Add` as an
explicit field instead of mutating `State.config`, because
`State.config` is `&'static Config` — the CLI override merge happens
at the CLI layer and the fully-resolved value lands on the install
pipeline.

Tests:

- `workspace_yaml/tests.rs` — yaml round-trip across `os` / `cpu` /
  `libc`, omission, and partial-axis cases.
- `installability/tests.rs` — `supportedArchitectures` widens the
  accept set so a darwin-only package stays on a linux host when the
  user lists `darwin`; conversely, listing `linux` keeps the
  darwin-only package skipped (no implicit host include).

Out of scope (umbrella slice 3+): `.modules.yaml.skipped` persistence,
current-lockfile diffing for `removeOptionalDependenciesThatAreNotUsed`,
and `--no-optional` plumbing.

Closes #453.

Origin/main grew a `path: Option<String>` field on
`TarballResolution` (PR #451, `feat(git-fetcher): install
git-hosted tarballs via preparePackage + packlist`). That PR
landed after #439 (`feat(package-is-installable): platform +
engine check`) and missed updating the `TarballResolution`
literal in `crates/package-manager/src/installability/tests.rs`,
so any PR opened against current main inherits a build break in
the `package-is-installable` test build (#445 hit the same
failure). My own `peer_dependency_in_lockfile_surfaces_unsupported`
test in `crates/real-hoist/src/tests.rs` had the same gap.

Add `path: None` to both literals so the test builds compile
against the post-#451 `TarballResolution` shape.

Includes the rebase of feat/438-slice-3b onto current origin/main.

…bc detection (#434 slice 2)

Slice 1 (#439) wired `pacquet-package-is-installable` into the install
pipeline but the `SupportedArchitectures` input was always `None`, so
every install behaved as if the user had opted into "host triple only".
This slice closes that gap by wiring the three upstream input sources
pinned at pnpm/pnpm@94240bc046:

- `Config.supported_architectures` deserialized from
  `pnpm-workspace.yaml`, mirroring
  `getOptionsFromRootManifest.ts`.
- `--cpu` / `--os` / `--libc` flags on `install` / `add`,
  multi-valued, comma-separated. Per-axis CLI override replaces the yaml
  axis wholesale, matching `overrideSupportedArchitecturesWithCLI.ts`.
- Real Linux libc detection — replaces the slice 1 `"unknown"` stub
  with a `/lib*/ld-musl-*` probe cached via `OnceLock`. Mirrors
  `detect-libc.familySync()` semantics: `"musl"` / `"glibc"` on
  Linux, `"unknown"` everywhere else. No new dep — open-coded
  `read_dir` is cheaper than spawning `ldd --version` and works in
  slim containers without `ldd` on PATH.

Threaded through `Install` / `InstallFrozenLockfile` / `Add` as an
explicit field instead of mutating `State.config`, because
`State.config` is `&'static Config` — the CLI override merge happens
at the CLI layer and the fully-resolved value lands on the install
pipeline.

Tests:

- `workspace_yaml/tests.rs` — yaml round-trip across `os` / `cpu` /
  `libc`, omission, and partial-axis cases.
- `installability/tests.rs` — `supportedArchitectures` widens the
  accept set so a darwin-only package stays on a linux host when the
  user lists `darwin`; conversely, listing `linux` keeps the
  darwin-only package skipped (no implicit host include).

Out of scope (umbrella slice 3+): `.modules.yaml.skipped` persistence,
current-lockfile diffing for `removeOptionalDependenciesThatAreNotUsed`,
and `--no-optional` plumbing.

Closes #453.

…bc detection (#434 slice 2) (#456)

Slice 2 of the [#434 umbrella](#434) (`Proper optionalDependencies support`). Slice 1 (#439) wired `pacquet-package-is-installable` into the install pipeline but the `SupportedArchitectures` input was always `None`, so every install behaved as if the user had opted into \"host triple only\". This PR closes that gap and replaces the slice 1 libc stub with a real Linux probe.

Mirrors the three upstream input sources pinned at [pnpm/pnpm@94240bc046](https://github.com/pnpm/pnpm/tree/94240bc046):

- **`Config.supported_architectures` from `pnpm-workspace.yaml`** — same shape as upstream's `getOptionsFromRootManifest.ts` ([`config/reader/src/getOptionsFromRootManifest.ts#L23`](https://github.com/pnpm/pnpm/blob/94240bc046/config/reader/src/getOptionsFromRootManifest.ts#L23)).
- **`--cpu` / `--os` / `--libc` flags on `install` / `add`** — multi-valued, comma-separated. Per-axis CLI override replaces the yaml axis wholesale, matching upstream's [`overrideSupportedArchitecturesWithCLI.ts`](https://github.com/pnpm/pnpm/blob/94240bc046/config/reader/src/overrideSupportedArchitecturesWithCLI.ts).
- **Real Linux libc detection** — replaces the slice 1 `\"unknown\"` stub with a `/lib*/ld-musl-*` probe cached via `OnceLock`. Mirrors `detect-libc.familySync()` semantics: `\"musl\"` / `\"glibc\"` on Linux, `\"unknown\"` everywhere else. No new dep — open-coded `read_dir` is cheaper than spawning `ldd --version` and works in slim containers without `ldd` on PATH.

Threaded through `Install` / `InstallFrozenLockfile` / `Add` as an explicit field instead of mutating `State.config`, because `State.config` is `&'static Config` — the CLI override merge happens at the CLI layer and the fully-resolved value lands on the install pipeline.

## Test plan

Unit tests added; full suite ran via `just ready` + `cargo doc --no-deps` + `taplo format --check` + `just dylint`.

- [x] `cargo test -p pacquet-config workspace_yaml::tests::*supported_architectures*` — yaml round-trip across `os` / `cpu` / `libc`, omission, partial-axis (3 tests).
- [x] `cargo test -p pacquet-package-manager --lib installability::tests::supported_architectures*` — `supportedArchitectures` widens the accept set so a darwin-only package stays on a linux host when the user lists `darwin`; conversely, listing `linux` keeps the darwin-only package skipped (no implicit host include).
- [x] `just ready` — typos + fmt + check + nextest (792 tests pass) + clippy.
- [x] `taplo format --check`.
- [x] `just dylint` (perfectionist).
- [x] `RUSTDOCFLAGS=\"-D warnings\" cargo doc --no-deps --workspace`.

## Out of scope

- `.modules.yaml.skipped` persistence (umbrella slice 3).
- Current-lockfile diffing for `removeOptionalDependenciesThatAreNotUsed` (umbrella slice 6).
- `--no-optional` plumbing (umbrella slice 5).

Closes #453.

Address PR #445 review feedback after the rebase pulled in #439
(installability check / `SkippedSnapshots`) + #443 (workspace
install).

- **Honor `SkippedSnapshots` in the hoist pass** (Copilot, real
  bug). `install_frozen_lockfile.rs` was passing
  `HashSet::new()` to `HoistInputs.skipped` even though
  `compute_skipped_snapshots` already produces the
  optional+platform-incompatible skip set earlier in the same
  function. Effect of the fix: hoist no longer creates symlinks
  to slots that were never extracted, and an alias that would
  have been claimed by a skipped snapshot can now be claimed by
  a non-skipped sibling at a deeper level. The `HoistInputs`
  shape stays `&HashSet<PackageKey>` so `hoist.rs` doesn't
  depend on `installability`; the cheap conversion happens at
  the call site.

- **Refresh `build_direct_deps_by_importer` doc comment**
  (Copilot). Doc said "currently only installs the root importer"
  — wrong since #443 (workspace install) and the follow-up that
  switched the call site to pass the full `importers` map.
  Updated to describe the current per-importer walk and the
  `link:` filter inside the loop.

- **Refresh `skipped_optional_deps` known-failure reason**
  (Copilot). The reason claimed pacquet doesn't compute skip
  sets — wrong since #439. The actual gap is the test fixture:
  upstream's `@pnpm.e2e/not-compatible-with-any-os` isn't in
  pacquet's mocked registry, so the end-to-end skip-then-hoist
  path can't be exercised yet. Reason updated to point at the
  fixture gap, not the (already-implemented) skip computation.

Tests: `just ready` 902/902 pass; `cargo doc --workspace --no-deps`
with `RUSTDOCFLAGS="-D warnings"` clean; `just dylint` clean.
The 36 CLI hoist tests + 12 unit tests still pass with the real
skip set threaded through.

Out of scope: Copilot's comment about `satisfies_package_manifest`
only validating the root importer is about main's #447 freshness
check (not this PR's hoist work). Worth a follow-up issue but
not addressed here.

Pull in 28 commits from upstream main, including the
`pacquet-npmrc` → `pacquet-config` rename (#420) plus features:
- supportedArchitectures + --cpu/--os/--libc (#456)
- frozen-lockfile (#442, #443, #447, #450)
- git-fetcher (#436 / #446, #451, #454)
- side-effects cache (#421 / #422, #423, #424)
- real-hoist + global-virtual-store (#432 / #438 / #444, #449, #452)
- patchedDependencies + allow-builds (#425, #427, #428)
- engine/platform installability (#434 / #439)

Conflicts resolved:
- `crates/npmrc/` files migrated under the renamed
  `crates/config/` directory; `Npmrc` → `Config` everywhere
  except `NpmrcAuth` (which keeps the `.npmrc`-domain name).
- `Config::current` reads the env-var DI generic `Api: EnvVar`
  for ${VAR}-substitution in `.npmrc`. Production turbofish in
  `cli_args.rs` is `Config::current::<RealApi, _, _, _, _>(...)`.
- Two-phase `NpmrcAuth::apply_*` retained so default-registry
  creds key at the yaml-resolved registry URL.
- New `Config::auth_headers` field plumbed through
  `install_package_by_snapshot`'s `DownloadTarballToStore`.
- Tests under `crates/config/src/workspace_yaml/tests.rs`
  pick up the new ParseYaml unit test added on this branch.

… slice 3) (#467)

## Summary

Slice 3 of the [#434 umbrella](#434) (`Proper optionalDependencies support`). Slice 1 (#439) computed a `SkippedSnapshots` set on every frozen-lockfile install but never serialized it; slice 2 (#456) closed the input side. This PR closes the loop by mirroring three upstream sites pinned at [pnpm/pnpm@94240bc046](https://github.com/pnpm/pnpm/tree/94240bc046):

- **Write** — `Modules.skipped` was declared at [`crates/modules-yaml/src/lib.rs:197`](https://github.com/pnpm/pacquet/blob/b13b8180/crates/modules-yaml/src/lib.rs#L197) with sort-on-write but always serialized empty. `build_modules_manifest` now takes `&SkippedSnapshots` and produces the depPath string list pnpm writes at [`installing/deps-installer/src/install/index.ts:1625`](https://github.com/pnpm/pnpm/blob/94240bc046/installing/deps-installer/src/install/index.ts#L1625).

- **Seed** — `install_frozen_lockfile::run` reads `.modules.yaml.skipped` before computing the in-memory set and passes the parsed `PackageKey`s as the seed to `compute_skipped_snapshots`. Mirrors upstream's load at [`installing/read-projects-context/src/index.ts:79`](https://github.com/pnpm/pnpm/blob/94240bc046/installing/read-projects-context/src/index.ts#L79) plus the early return at [`deps/graph-builder/src/lockfileToDepGraph.ts:194`](https://github.com/pnpm/pnpm/blob/94240bc046/deps/graph-builder/src/lockfileToDepGraph.ts#L194).

- **Short-circuit** — `compute_skipped_snapshots` returns the seed verbatim on the fast path (no constraints in the lockfile) and, on the slow path, skips the per-snapshot re-check for keys already in the seed without emitting a duplicate `pnpm:skipped-optional-dependency` event. Non-seeded snapshots still re-run `package_is_installable`, covering the \"host arch changed since last install\" case upstream guards at [`:206-215`](https://github.com/pnpm/pnpm/blob/94240bc046/deps/graph-builder/src/lockfileToDepGraph.ts#L206-L215).

`InstallFrozenLockfile::run` now returns a small `InstallFrozenLockfileOutput { hoisted_dependencies, skipped }` struct so `Install::run` can thread both into `.modules.yaml`. Read errors on `.modules.yaml` degrade to an empty seed — the file is a cache artifact, not authoritative.

…e payload (#434 slice 4) (#474)

Slice 4 of the [#434 umbrella](#434) (`Proper optionalDependencies support`). Slices 1–3 (#439, #456, #467) close the installability-driven skip path; this slice adds the second skip pathway upstream handles in the frozen install: an `optional: true` snapshot whose **fetch / extract** step fails at install time must not abort the install. Local-materialization errors (`CreateVirtualDir`) and config-shape errors still abort even for optional snapshots — matching upstream's `linkPkg` path which sits outside the catch.

Mirrors the silent catch sites at [`deps/graph-builder/src/lockfileToDepGraph.ts:294-298`](https://github.com/pnpm/pnpm/blob/94240bc046/deps/graph-builder/src/lockfileToDepGraph.ts#L294-L298) and [`installing/deps-restorer/src/index.ts:912-921`](https://github.com/pnpm/pnpm/blob/94240bc046/installing/deps-restorer/src/index.ts#L912-L921):

```ts
try { ... fetch ... }
catch (err) { if (pkgSnapshot.optional) return; throw err }
```

## Changes

- **Reporter**: `SkippedOptionalPackage` becomes a `#[serde(untagged)]` enum with `Installed { id, name, version }` and `ResolutionFailure { name?, version?, bareSpecifier }` variants. Models upstream's discriminated union at [`core-loggers/src/skippedOptionalDependencyLogger.ts:10-31`](https://github.com/pnpm/pnpm/blob/94240bc046/core/core-loggers/src/skippedOptionalDependencyLogger.ts#L10-L31). The new variant is wire-shape-only in slice 4 — pacquet has no resolver yet, but a future resolver port at the upstream emit site ([`resolveDependencies.ts:1376-1383`](https://github.com/pnpm/pnpm/blob/94240bc046/installing/deps-resolver/src/resolveDependencies.ts#L1376-L1383)) lands without re-touching this type.
- **`SkippedSnapshots`**: gains a `fetch_failed` subset disjoint from the existing `installability` subset. `contains` / `iter` return the union (downstream consumers treat both as absent); `iter_installability` returns only the persistent subset that `.modules.yaml.skipped` records, matching upstream's behavior of never updating `opts.skipped` at the fetch-failure catch sites.
- **`CreateVirtualStore`**: cold-batch dispatch swallows per-snapshot errors when `snapshot.optional` is true **and** the error is fetch-side. A new `is_fetch_side_failure` helper restricts the swallow to `InstallPackageBySnapshotError::DownloadTarball` and `::GitFetch` — the same surface upstream wraps in `storeController.fetchPackage`. Local-materialization (`CreateVirtualDir`) and config-shape errors (`MissingTarballIntegrity` / `UnsupportedResolution`) propagate even for optional snapshots, matching upstream's `linkPkg` path which sits outside the catch. Side benefit: the warm-batch path (which only surfaces `CreateVirtualDir` errors) stays consistently abort-on-error without a parallel swallow site. The per-install `fetch_failed: HashSet<PackageKey>` rides out on `CreateVirtualStoreOutput`.
- **`InstallFrozenLockfile::run`**: folds the returned `fetch_failed` into its mutable `SkippedSnapshots` so downstream phases (`SymlinkDirectDependencies`, `LinkVirtualStoreBins`, `BuildModules`, hoist) treat the dropped snapshots as absent through the same gate they already use for installability skips.

## Test plan

- [x] `reporter::tests::skipped_optional_resolution_failure_event_matches_pnpm_wire_shape` — full payload (`bareSpecifier` camelCase, no `id`).
- [x] `reporter::tests::skipped_optional_resolution_failure_omits_absent_name_and_version` — optional-field shape.
- [x] `install::tests::frozen_install_silently_swallows_unreachable_optional_tarball` — ports [`installing/deps-restorer/test/index.ts:340-360`](https://github.com/pnpm/pnpm/blob/94240bc046/installing/deps-restorer/test/index.ts#L340-L360): unreachable optional tarball (`http://127.0.0.1:1/...`), install succeeds, slot not created, `.modules.yaml.skipped` left empty. **Test-the-test verified** by inverting the `optional` guard in the cold-batch match arm. Runs with `enable_global_virtual_store = false` so the slot-path assertion targets the legacy layout.
- [x] `install::tests::frozen_install_propagates_non_optional_fetch_failure` — pins the polarity: same fixture, non-optional snapshot, install must error.
- [x] `just ready` (typos + fmt + check + nextest + clippy).
- [x] `taplo format --check`.
- [x] `just dylint` (perfectionist).
- [x] `RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --workspace`.

## Out of scope

- Resolver-side `resolution_failure` emit site — needs a resolver, tracked separately.
- `--no-optional` / `--omit=optional` plumbing — umbrella slice 5.
- Surfacing fetch failures to the reporter on the frozen path — upstream itself is silent here; only the resolver-side emit fires `pnpm:skipped-optional-dependency`.

Closes #471.

…#485)

Slice 5 of the [#434 umbrella](#434) (`Proper optionalDependencies support`). Slices 1–4 (#439, #456, #467, #474) handle installability + the fetch-failure swallow. This slice closes the user-facing `--no-optional` flag: the CLI flag already exists and threads through `Install::dependency_groups`, but only `SymlinkDirectDependencies` and the on-disk `included` field actually honored it. The rest of the install pipeline walked `optional_dependencies` unconditionally, so the optional subtree was still extracted, linked, and built.

Mirrors upstream's depNode filter at [`installing/deps-installer/src/install/link.ts:109-111`](https://github.com/pnpm/pnpm/blob/94240bc046/installing/deps-installer/src/install/link.ts#L109-L111):

```ts
if (!opts.include.optionalDependencies) {
  depNodes = depNodes.filter(({ optional }) => !optional)
}
```

## Changes

- **`SkippedSnapshots`**: gains a third disjoint subset `optional_excluded` alongside `installability` (slice 1) and `fetch_failed` (slice 4). The existing `contains` / `iter` already return the union, so every downstream consumer that filters by skip-set (`CreateVirtualStore`, `SymlinkDirectDependencies`, `BuildModules`, hoist, `link_bins`) now respects `--no-optional` through the same gate. `iter_installability` still returns only the persistent subset for `.modules.yaml.skipped` writes — `--no-optional` exclusions are transient (matching upstream's behavior of never updating `opts.skipped` at the filter site).
- **`InstallFrozenLockfile::run`**: iterates the lockfile snapshots once and inserts every `snap.optional == true` entry into the new subset when the dispatch list doesn't contain `DependencyGroup::Optional`. The `SnapshotEntry::optional` flag is set by the resolver only when the snapshot is reachable **exclusively** through optional edges, so a snapshot reachable through any non-optional edge carries `optional: false` and survives the filter — covers [`optionalDependencies.ts:712`](https://github.com/pnpm/pnpm/blob/94240bc046/installing/deps-installer/test/install/optionalDependencies.ts#L712) (`dependency that is both optional and non-optional is installed`).