perf(cache): batch writes and validate hydration

[lzehrung]

Summary

• Batch cache writes and hydrate snapshot and bloom artifacts on warm loads.
• Reject stale hydrated artifacts through per-file signature validation and schema gates.

Verification

• Focused cache invalidation and cross-file reference recovery regressions passed.
• Final hydration review approved unchanged warm-cache reuse and stale-artifact rejection.

[lzehrung]

Superseded by #261, which consolidates cache batching and hydration validation on the portable-cache base.