fn FindFilesInFlight() -> &'static Mutex<HashMap<FindFilesCacheKey, Arc<Notify>>>Expand description
Single-flight registry: keys with a walk currently in progress
share the same Notify so concurrent callers awaiting the same
(folders, include, exclude, cap, flags) don’t each kick off
their own filesystem walk.
Why: log audit (20260501T053137) showed 1023 findFiles calls
during one extension-boot session, with the cache hit rate
at ~67% (687 hits, 333 misses). The 333 misses fired BEFORE
the first walker for any given key populated the cache, so
each one independently re-walked the same tree. With the
single-flight guard the leader walks once, every concurrent
follower awaits, then reads the freshly-populated cache.