Avoid full scan cache rewrites for targeted scans

[jmqd]

Targeted scan_paths calls now reuse and refresh cache entries, but any changed requested path still serializes the whole cache file. Small dirty reviews in large repos can pay whole-cache clone/serialization costs. Use a patchable/per-file cache layout or gate full rewrites with measured thresholds.

[jmqd]

Context: local commits now let scan_paths reuse/refresh cache entries and avoid cloning the loaded cache map. Remaining cost: changed targeted paths still write the whole cache file. Verification for current cache changes: scanner cache/path tests passed; just check passed with 979 tests, clippy, and fmt.