feat(library): Add rating sync with file tags (FMPS_RATING standard)

[SimonDedman]

Adds support for saving and loading track ratings to/from file metadata tags, addressing a long-requested feature (#9477).

Overview

This implementation uses the FMPS_RATING standard (0.0-1.0 scale), which is compatible with Strawberry, Foobar2000, Quod Libet, and other music players that follow the freedesktop.org Media Player Specs. The feature is non-destructive (does not overwrite POPM/WMP ratings) and user opt-in (disabled by default).

Key design decisions:

• Non-destructive: Does not overwrite POPM (Windows Media Player) or other rating formats
• User opt-in: Disabled by default, configurable in Preferences > Library
• Bidirectional: Separate options for import and export

Changes

TagLib Layer (Phase 1)

Rating import/export functions for all supported formats:

• ID3v2 (MP3): TXXX:FMPS_Rating frame write, POPM fallback read
• Xiph (FLAC/OGG): FMPS_RATING Vorbis comment
• APE (WavPack/APE): FMPS_Rating item
• MP4 (M4A): ----:org.freedesktop.FMPS:FMPS_Rating atom

Preferences UI (Phase 2)

Two new checkboxes in Library preferences under "Track Metadata Synchronization":

• Export rating to file tags - Saves Mixxx ratings to files on metadata export or quit
• Import rating from file tags - Loads ratings when tracks are selected/imported

Integration (Phase 3-4)

• MetadataSourceTagLib: New importRating() and exportRating() virtual methods
• Track::exportMetadata(): Exports rating after metadata when preference enabled
• SoundSourceProxy::updateTrackFromSource(): Imports rating during track metadata update
• TrackCollectionManager::exportTrackMetadataBeforeSaving(): Respects rating export preference
• RatingSyncWorker: Background worker for syncing ratings when opening playlists/crates

Rating Format Support

Format	Tag Type	Read	Write
MP3	TXXX:FMPS_Rating	✅	✅
MP3	POPM (fallback)	✅	❌
FLAC/OGG	FMPS_RATING	✅	✅
M4A	FMPS_Rating atom	✅	✅
WavPack	FMPS_Rating (APE)	✅	✅

Conversion Scale

Mixxx	FMPS	Display
0	0.0	No rating
1	0.2	★
2	0.4	★★
3	0.6	★★★
4	0.8	★★★★
5	1.0	★★★★★

Testing

• ✅ Tested bidirectional sync with Strawberry music player
• ✅ Ratings export correctly to FMPS_Rating format on quit
• ✅ Ratings import correctly when selecting tracks in library
• ✅ Verified with kid3: exported rating shows FMPS_Rating = 1.0 for 5★

Screenshots

The new preferences options are located in Preferences > Library > Track Metadata Synchronization section

Test plan

• Enable "Export rating to file tags" preference
• Rate a track 5 stars in Mixxx
• Close Mixxx
• Verify file has FMPS_Rating = 1.0 (use exiftool, kid3, or similar)
• Enable "Import rating from file tags" preference
• Add a file with existing FMPS_Rating to library
• Verify rating appears in Mixxx

Closes #9477

[JoergAtGithub]

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[acolombier]

Some high level comments before doing a more deep dived review:

• Please make sure to use smart pointer
• Use snake_case for new config keys
• Please explain why we need a custom worker as opposed to use the same metadata runner
• We will need to have some test to ensure the supported file types are indeed well exported

[SimonDedman]

Thanks for the review! All four items addressed in the latest push:

1. Smart pointer: Removed RatingSyncWorker entirely, so the raw pointer is gone. The existing SoundSourceProxy::updateTrackFromSource() pipeline already handles rating import via syncParams.importRatingFromFile, making the custom worker unnecessary.

2. snake_case config keys: Renamed to export_rating_to_file_tags and import_rating_from_file_tags.

3. Why custom worker?: See master sync first merge #1 — we don't need one. Removed it along with all references in baseplaylistfeature and cratefeature.

4. Tests: Added ratingexportimport_test.cpp with:

   • Parameterized round-trip test across all 8 supported formats (MP3, FLAC, OGG, Opus, M4A, WAV, AIFF, WavPack)
   • All rating values (1–5) round-trip test
   • Clear rating test (export 0, verify no rating returned)
   • No-rating-initially test (fresh file returns nullopt)

[SimonDedman]

Some high level comments before doing a more deep dived review:

* Please make sure to use smart pointer

* Use `snake_case` for new config keys

* Please explain why we need a custom worker as opposed to use the same metadata runner

* We will need to have some test to ensure the supported file types are indeed well exported

Cheers for the notes Antoine. PR should be fixed now. Cheers!

[SimonDedman]

@acolombier Antoine any chance someone could have a peek at this?