Follow-up to #434 (umbrella, closed). Slices 1–3 (#439, #456, #467) handle installability-driven skips end to end. This slice closes a different swallow path: an optional: true snapshot whose tarball / metadata / extract step fails at install time must not abort the install. The non-optional case still aborts.

Scope

Mirror pnpm/pnpm@94240bc046's two frozen-path catch sites:

• lockfileToDepGraph.ts:286-298: wraps storeController.fetchPackage; if (pkgSnapshot.optional) return; throw err;.
• deps-restorer/src/index.ts:912-921: wraps the deferred depNode.fetching() import; same if (depNode.optional) return; throw err;.

Both sites are silent — no pnpm:skipped-optional-dependency event is emitted at fetch time on the frozen path. The corresponding upstream test at deps-restorer/test/index.ts:340-360 only asserts that the install completes and the other deps land — it never inspects the reporter mock.

For pacquet the equivalent surface is InstallPackageBySnapshot::run inside CreateVirtualStore. Today every fetch / extract error there propagates unconditionally as CreateVirtualStoreError::InstallPackageBySnapshot(...). This slice:

• Wraps the per-snapshot fetch / extract dispatch so an optional: true snapshot's TarballError / WriteCasFileError / similar is silently dropped (no virtual-store slot created, no symlink, install continues).
• Non-optional failures still surface unchanged.
• The skip set is not updated for fetch failures — matches upstream behavior at the graph-builder site (only installability failures land in opts.skipped at :213).

ResolutionFailure sibling payload

The doc comment at crates/reporter/src/lib.rs:546-562 already flags that resolution_failure carries a different payload shape ({ name?, version?, bareSpecifier }) than the other three reasons ({ id, name, version }). Upstream defines the discriminated union explicitly at core-loggers/src/skippedOptionalDependencyLogger.ts:10-31.

Land the variant now — either via a sibling SkippedOptionalDependencyLog enum or #[serde(untagged)] on SkippedOptionalPackage. The actual emit site is resolver-only (resolveDependencies.ts:1376-1383) and pacquet has no resolver yet, so the variant is wire-shape-only in slice 4. Slice 7 (ignoredOptionalDependencies) and any future resolver-side work consume it.

Tests to port

• installing/deps-restorer/test/index.ts:340-360 skipping optional dependency if it cannot be fetched — broken-URL optional snapshot, sibling non-failing snapshots, install completes, sibling slots present.

Out of scope

• Resolver-side resolution_failure emit site — needs a resolver, tracked separately.
• Build-failure swallow — already shipped (feat(executor): swallow optional-dep build failures and report via pnpm:skipped-optional-dependency (#397) #419).
• --no-optional / --omit=optional plumbing — umbrella slice 5.

Upstream pin

pnpm/pnpm@94240bc046. Pacquet umbrella slice 1 in #439, slice 2 in #456, slice 3 in #467; this is umbrella slice 4.

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