fix: self-heal stale search indexes

[aldegad]

Summary

• Detect archive/index revision drift and reject stale or orphan semantic generations before search.
• Rebuild semantic generations atomically from the canonical archive, preserve the last committed generation on failure, and return structured JSON errors with non-zero exits.
• Treat BM25 input as literal user text and document cross-room top-k expansion with stable chat IDs.

Tests

• cargo +1.91.0 fmt --all -- --check
• cargo +1.91.0 clippy --all-targets -- -D warnings
• KATOK_EMBEDDER=local-test cargo +1.91.0 test --all-targets
• cargo +1.91.0 publish --dry-run
• python3 scripts/verify_release_config.py