Write MUSICBRAINZ_TRACKID to file tags on "Complete tags automatically"

[VictorVow]

Problem

"Complete tags automatically" retrieves the MusicBrainz recording UUID via AcoustID, uses it to query the MusicBrainz API for metadata, then silently discards it. The UUID never enters the Result struct and cannot reach the tag-write path. Only standard metadata (title, artist, album, year, track number) is written to files.

Solution

Wire the recording UUID through the full pipeline so it is written to file tags alongside the other metadata.

Changes

src/musicbrainz/musicbrainzclient.h / .cpp

• Result gains a musicbrainz_recording_id_ field
• MbIdRequestFinished receives the mbid it looked up (already captured in the lambda) and stamps it onto every Result produced from that request

src/musicbrainz/tagfetcher.cpp

• TagsFetched copies musicbrainz_recording_id_ onto the Song built from each Result

src/dialogs/trackselectiondialog.cpp

• SaveData includes musicbrainz_recording_id in the set of fields copied to the file on user confirmation

src/tagreader/tagreadertaglib.cpp

• ID3v2 (MP3, WAV, AIFF): TXXX frame with description MUSICBRAINZ_TRACKID
• Vorbis comments (FLAC, OGG, Opus): MUSICBRAINZ_TRACKID field
• APE tags (WavPack, Musepack, Monkey's Audio): MUSICBRAINZ_TRACKID item
• MP4/iTunes (M4A, AAC): ----:com.apple.iTunes:MusicBrainz Track Id
• ASF/WMA: MusicBrainz/Track Id attribute
• Added TXXX read path for MUSICBRAINZ_TRACKID so tags written by this code are read back into the database on the next scan

Notes

• All writes are guarded with !isEmpty() — no empty tags are written
• The existing UFID read path for musicbrainz_recording_id is preserved for files tagged by external tools (Picard, beets)
• The ASF write uses the existing SetAsfAttribute helper which already handles empty-string removal

[jonaski]

Thanks!