clementine-0001: LibraryWatcher ScanSubdirectory FindSongByPath O(F*S) +
files_on_disk.contains O(S*F) — linear scan with TODO comment, fix with
HashMap + HashSet. HIGH severity, 250x at N=1000.
clementine-0002: SongSender indexOf(s) O(N^2) in SendAlbum/SendPlaylist/
SendUrls loops — fix with integer counter + QSet for requested_ids.
MEDIUM severity, 500x at N=1000.
MPD: CLEAN — uses std::set, bitmask arrays, std::map throughout.
Rhythmbox: CLEAN — uses g_hash_table for all membership checks.
4/4 unit tests PASS.
