…e payload (pnpm#434 slice 4) (pnpm#474)

Slice 4 of the [pnpm#434 umbrella](pnpm/pacquet#434) (`Proper optionalDependencies support`). Slices 1–3 (pnpm#439, pnpm#456, pnpm#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 pnpm#471.

…e payload (pnpm#434 slice 4) (pnpm#474)

Slice 4 of the [pnpm#434 umbrella](pnpm/pacquet#434) (`Proper optionalDependencies support`). Slices 1–3 (pnpm#439, pnpm#456, pnpm#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 pnpm#471.