feat(metadata): add advanced field policy

[flamboh]

What changed

• add album artist, disc number, composer, BPM, and comment across metadata snapshots, sparse patches, hydration, editor sessions, export, and MP3 I/O
• add an off-by-default Advanced Metadata gate and one backward-compatible global metadata-link policy
• centralize album-to-track policy application for album operations, imports, moves, cover/number sync, hydration, and export
• preserve current library values when settings change; apply policy only at defined future synchronization/write seams
• preserve unknown ID3v2 frames, alternate comments, and byte-identical audio payloads during real mp3tag.js round trips
• upgrade untagged/ID3v1 inputs to ID3v2.4 when advanced fields require it
• add real-codec and policy regression coverage

Why

Advanced editor controls need a behavior-preserving metadata and synchronization foundation. This PR keeps visible behavior unchanged while making link semantics and advanced field persistence explicit.

Verification

• post-commit bun run typecheck
• post-commit bun run lint — 0 warnings/errors
• post-commit bun run test — 65 files, 412 tests passed
• git diff --check
• two fresh independent reviews; all data-loss and hydration findings fixed and re-reviewed

Known format constraint

Album artist, composer, BPM, and disc number have no ID3v1 representation. When they are written to an ID3v1-only or untagged MP3, Tagium adds ID3v2.4 while preserving the audio payload.

This PR intentionally targets codex/format-aware-audio-core (PR #119); its format identity and naming seam are a genuine dependency.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	tagium	b8109a8	Commit Preview URL Branch Preview URL	Jul 17 2026, 01:25 PM

[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit b8109a8.}

[flamboh]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat(audio): add waveform preview playback #127
• feat(ui): add mobile workspace navigation #125
• feat(export): confirm bulk downloads #123
• feat(metadata): reopen cleanup suggestions #121
• feat(ui): polish workspace states #120
• feat(metadata): add advanced editor #124
• feat(metadata): add advanced field policy #122 👈 (View in Graphite)
• refactor(audio): make formats explicit #119
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[flamboh]

Superseded together with #124 by the combined advanced metadata rebuild in #137.