feat(memory): add a rebuildable local lexical index for Mate topics

[robinbraemer]

Outcome

Add a local rebuildable lexical/BM25 index for bounded Mate memory topics without changing Markdown authority.

Scope

• Chunk topic bodies with metadata and stable source hashes.
• Evaluate Bun/SQLite FTS5 and simpler in-process alternatives before selecting the smallest reliable derivative format.
• Build and update the index atomically after validated topic mutations.
• Support deterministic top-k search, type/scope filtering, and bounded recency signals.
• Fall back to the current selector when the index is missing, stale, corrupt, or incompatible.

Acceptance criteria

• Deleting the index and restarting rebuilds equivalent results from Markdown.
• Partial writes and Pod interruption cannot make stale results appear current.
• Index files are private to the Mate PVC and never copied to PostgreSQL or another Mate.
• Search and rebuild stay within declared CPU, memory, disk, and latency budgets.
• Lexical retrieval is evaluated against the baseline before becoming a default candidate.