Use MusicBrainz durations to infer song endings

[keithah]

Use MusicBrainz recording durations as a secondary heuristic for stale song metadata.

Problem: song-start metadata can linger beyond the real song, suppressing ad or spoken transcription.

Idea: after a song is identified, resolve its expected duration in the background and let the song state expire after duration plus tolerance unless fresh metadata/fingerprint evidence extends it.

Acceptance criteria:

• Known songs can expire from expected duration plus tolerance.
• Spoken/ad audio after a stale song marker becomes eligible for transcription.
• UI distinguishes inferred duration from direct stream metadata.
• Tests cover exact match, missing duration, version mismatch, and stale metadata fallback.

[keithah]

First issue #1 slice landed on main in 2a66aa7 (fix: Expire stale song metadata by duration).

What changed:

• Carried timed-metadata song state now checks matching cached AcoustID/MusicBrainz-style recording duration before it can continue suppressing transcript capture.
• Match is conservative: ISRC match, or exact normalized title + artist match. Different-recording metadata does not expire the carried song.
• If no cached duration exists, behavior falls back to the previous carried-song suppression path.
• Once the current chunk starts after song_start + cached_duration + 30s, the stale song is no longer treated as active for transcript suppression, so spoken/ad audio becomes eligible for transcription.

Verification:

• Red/green focused tests for known duration expiry, missing duration fallback, and different-recording mismatch.
• swift test --filter SoundingKitTests.StreamIngestPipelineSongTests (25 tests, 0 failures)
• swift test --filter SoundingKitTests.SongTimelinePersistenceTests --filter SoundingKitTests.AcoustIDLookupCacheTests --filter SoundingKitTests.AcoustIDEnrichmentTests --filter SoundingKitTests.StreamAppTimelineProjectionTests (53 tests, 0 failures)
• swift build --product sounding
• git diff --check
• Xcode Debug app build with .build/xcode-debug derived data

Remaining acceptance work: surface inferred-duration evidence in the UI/timeline so users can distinguish duration-expired song state from direct stream metadata.