Make scan cache freshness content-aware

[jmqd]

Scan cache reuse currently validates entries with mtime plus size. Same-size edits with preserved or coarse mtimes can reuse stale parsed blocks and hide unreviewed code. Add content-aware validation or an equivalent freshness check without reducing scan coverage.

[jmqd]

Context: scanner cache entries are keyed by FileStamp { modified_at, size }. A same-size rewrite with preserved/coarse mtime can reuse stale blocks. This turn removed an unnecessary 5ms cache regression sleep where the file size already changes, but the underlying same-size freshness risk remains. Verification for current local scanner work: just check passed with 979 tests, clippy, and fmt.