Cover computeReleaseInfo: channel selection, assisted gating, migration fallback

[selfcontained]

Summary

apps/server/src/release-info.ts owns the assistedRequired classification for the update path but had zero direct coverage — release-auto-check.test.ts mocks computeReleaseInfo away entirely. This was the top-ranked gap from the last test-enforcer coverage audit.

Adds apps/server/test/release-info.test.ts (24 tests, no product code changes):

• deriveCurrentTag chain — release-store tag wins (version info never consulted), trimmed v-prefix fallback from app version, plain-semver regex rejection of prerelease versions, null when nothing is known.
• Channel / latest-tag selection — stable skips prereleases while absoluteLatestTag keeps the newest overall; latest channel takes prereleases; unknown channel settings coerce to stable; all-prerelease stable case yields latestTag: null; gh release list failure falls back to sorted local git tags with the v-prefix filter.
• updateAvailable — false for unknown current tag / equal / older latest; true for newer, with release metadata loaded and the notes body kept out of the snapshot.
• Assisted gating — through the real release-metadata parser: required mode forces assistedRequired, recommended does not, appliesFrom above the current install skips the gate, and a malformed dispatch-update fence hard-fails with ok: false before any tarball work.
• Migration evaluation — pending manifests map through the real toSummary (no manifest internals leak), per-file errors join into migrationsError, an evaluator crash degrades into migrationsError + assistedRequired instead of a failed result, and no evaluation runs when no update is available.
• Progress emission — full step ordering including the byte-progress mapping, and the finally emit(null) contract on both success and failure paths.

All I/O boundaries are mocked (runCommand, settings, release store, evaluator) — nothing can touch real git/gh/network/~/.dispatch.

Validation

• 7-mutant battery: 7/7 killed (channel selection, currentTag guard, both extra assistedRequired terms, finally-null, fallback filter, store-tag preference); source restored byte-identical.
• pnpm run check ✅ · server unit suite 2390 passed / 8 skipped ✅ · web 541 ✅ · E2E 175 passed / 12 skipped ✅ · live lane 11 passed, zero leaked tmux sessions ✅
• Review agent: 0 must-fix, 2 should-fix (applied: vi.resetAllMocks, fully-typed evaluator payloads via satisfies PendingMigrationsResult), nits applied.

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