refactor(library): move detection matches on the audio hash, not size and duration (M400 #3914)

A file that comes back renamed or moved keeps its track row, and with
it its likes and play history, by being matched to the missing row it
replaces (#2528). Untagged files were matched on (file_size,
duration_ms), which was never a fingerprint. It could pair two
unrelated files that happened to share a byte count and a duration,
and it missed a file retagged in place, whose size changes. The only
defence was requiring a unique match and otherwise giving up.

Now there is a real identity. FindMissingTrackByAudioHash matches a
missing track by the SHA-256 of its encoded audio (track_fingerprints,
#3906). That survives a rename, a move and a retag, and only an
identical recording can match it. adoptMovedTrack takes the new file's
hash, which the scan already computes before adoption. The size and
duration query and fallback are removed outright, with no second path
(rule 22).

Unchanged:
- MBID first: it identifies the recording and survives a re-encode
  that even the hash does not
- a unique match is still required
- an absent hash is never looked up, so unhashable files cannot pair
  with each other

The test fake answers the hash lookup only for the hash it holds, so
the tests can tell adoption by identity apart from adoption by
coincidence. That includes the case the old pair got wrong: different
audio of equal size and duration is not adopted.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

internal/library/scanner.go

@@ -325,11 +325,11 @@ func (s *Scanner) scanFile(
 	// file_path and updates THAT row: same track id, likes and play history
 	// intact. Without this, renumbering an album forks every track on it.
 	//
-	// Runs here rather than earlier because the fingerprint needs the probed
-	// duration, and only for genuinely unknown paths — a known path is already
-	// the row we're going to update.
+	// Runs after fingerprinting because, for a file with no MBID, adoption matches
+	// on its audio hash (#3914), and only for genuinely unknown paths — a known
+	// path is already the row we're going to update.
 	if !knownTrack {
-		if s.adoptMovedTrack(ctx, q, path, info.Size(), durationMs, recordingMBID) {
+		if s.adoptMovedTrack(ctx, q, path, fp.streamSHA256, recordingMBID) {
 			// Count it as an update: the row existed, and reporting it as Added
 			// would overstate library growth on every reorganisation.
 			knownTrack = true