feat: reuse parsed snapshot manifests across scans

[manuzhang]

Summary

• reuse parsed manifest-list entries across separate SnapshotCache wrappers and scans
• store parsed entries in FileIO-owned state without changing Snapshot's public layout
• coalesce concurrent parsing and tie parsed-entry lifetime to metadata byte-cache eviction
• keep existing manifest spans valid after shared-cache eviction and expose them as immutable spans

Why

PR #875 avoids repeated storage reads for immutable metadata, but each new scan still reparses cached manifest-list bytes. This follow-up reuses the parsed ManifestFile entries while the corresponding byte-cache entry remains alive.

Parsed entries are not retained for oversized content that bypasses the byte cache, and failures are shared with current waiters without becoming permanent cached failures.

Dependency

Depends on #875. Until #875 merges, GitHub's comparison against main includes the lower metadata-content-cache commit; the isolated change is commit d93c92c.

Validation

• pre-commit run --show-diff-on-failure --all-files
• focused C++23 syntax checks for snapshot caching, snapshot expiration, and manifest writer tests

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