perf(docs): persist the doc-search BM25 index across sessions

[pradeepmouli]

Summary

Doc search (hybrid_doc_search_in_dir) currently rebuilds its entire DocBM25Index in memory on every query — loading all chunks from the store and re-tokenizing the whole corpus — while code search already persists its BM25 index to .infigraph/bm25_cache.bin. This PR gives doc search the same treatment:

• DocBM25Index gains save/load/docs(). The on-disk format mirrors code search's BM25Index::save idiom (leading version byte + length-prefixed LE binary).
• hybrid_doc_search_in_dir loads docs_bm25_cache.bin when it is at least as new as docs_embeddings.bin — the same mtime freshness anchor code search uses, so a doc reindex invalidates the cache automatically. On a cache hit the store.get_all_chunks() DB read is skipped too (the cache carries the chunk texts).
• Miss/stale/corrupt cache → rebuild in memory exactly as before, then a best-effort atomic save (pid-unique temp file + rename). A cache failure can never fail or change a search — results are identical with and without the cache.

Two deliberate hardenings over the idiom being mirrored:

• load is fully bounds-checked and returns Err (never panics) on truncated/corrupt input, since a bad cache must degrade to a rebuild.
• The temp-file name is per-process unique, so concurrent searches can't race on the same temp path.

Group/combined doc stores get their own cache automatically (keyed off the store's artifact dir; combined stores publish per-generation directories, so no cross-generation staleness is possible).

Testing

• 3 new unit tests: save/load round-trip preserves scores; empty/wrong-version/truncated inputs rejected without panic; atomic save leaves no temp file.
• 3 new integration tests against a real DocStore: first search writes the cache and repeat results are identical; corrupt cache falls back silently and is rewritten; stale cache (newer embeddings) is rebuilt.
• Full infigraph-docs suite: 93/93 passing on this branch (rebased on current main); cargo fmt/clippy -D warnings/cargo check --workspace clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SjmvwHuwV5r7ZeZpJLp5oR