`src/middleware/memoize.rs` — `MemoizingMiddleware`

A per-turn-window cache for read-class tools, invalidated by write-class tools via declared paths.

Key items

• MemoizingMiddleware::new(tools, write_tools, path_extractor, ttl_turns) — all four explicit; ttl 0 caches nothing; NoopPathExtractor disables path invalidation.
• PathExtractor::paths(tool, input) -> Vec<String> — matched verbatim; be generous (over-returning only over-invalidates).
• Cache key: tool name + hash of canonicalized input JSON; entries carry result + turn + paths.

Behavior notes

• Errors are never cached; write results are never cached; a failed write doesn’t invalidate (filesystem unchanged).
• A hit appends [cached], carries the requesting call’s id, and replays the original duration (health stats track real latency).
• Concurrency-safe under parallel dispatch: an epoch check prevents a computed result from outliving a concurrent invalidation.
• No capacity limit / no LRU — pair with a short TTL for long sessions.
• serde_json’s preserve_order feature anywhere in your tree breaks key-order canonicalization.

Deep dive: Middleware.