feat(editor): drums become a type:"drums" arrangement (model foundation)

[ChrisBeWithYou]

Stacked on #335 (needs arrKind / _arrTypeKind / the type round-trip). Review/merge #335 first; I'll retarget this to main after.

What & why

The foundation for multiple drum charts (the drums-as-arrangements arc). The single drum tab has always lived entirely outside S.arrangements[] as a lone off-array singleton edited through a global mode — the one instrument that isn't an ordinary arrangement. This gives it a home in the list as a derived type:"drums" arrangement (via the #335 instrument-type-as-data seam), the substrate multiple drum charts grow from.

Byte-identical: no pack change, no UI change, drum-editor undo untouched. routes.py untouched.

How

New leaf src/drum-arrangement.js:

• syncDrumArrangement(S) materializes / updates / removes the drums arrangement so its .drumTab payload is S.drumTab — the same object reference, so every existing S.drumTab reader/mutator and every drum undo command (which hold references into S.drumTab.hits) keep working unchanged. Appends (never inserts), so existing arr indices / arr:<idx> keys are stable. Called at every S.drumTab (re)assignment: load migration, GP/MIDI import + empty-add, delete + its undo-restore.
• isDrumArrangement keys on the authored type alone (normalized), not arrKind — arrKind would name-infer and wrongly catch a pitched part a user literally named "Drums", then hide/drop it.
• pitchedArrangementCount / clampAwayFromDrums / pitchedIndexOf keep index math correct now that S.arrangements[] can hold a drums entry: the remove-last-arrangement guard counts pitched parts, S.currentArr never lands on drums, and the /remove-arrangement backend index maps to its pitched-only position (the backend manifest has no drums arrangement).

Byte-identical surfaces (drums bridged to the legacy paths; promoting them to arr:<idx> + drum-edit-via-selection is the follow-up):

• Save (_buildSaveBody, editorBuild): the drums arrangement is excluded from body.arrangements — drums still persist as the song-level drum_tab, so the built pack is byte-identical (and no drums entry reaches arrangements[], where an old core would fretted-grade it as garbage).
• Tracks targets / Parts view / band roster / pitched switcher: each skips the drums arrangement so it isn't listed twice — drums stay the legacy 'drums' target/key.

Tests

tests/drum_arrangement.test.mjs (+15): the sync state machine (materialize/update/remove/idempotent/append/same-ref/degrade), the remove→restore undo round-trip, byte-identical load→save, the "Drums"-named-but-untyped safety case (survives save, keeps its arr target), no duplicate tracks/band rows, and the index helpers incl. the interspersed-drums backend index.

Gates: JS 297/0 · lint 0 err / 3 baseline · routes.py untouched (no pytest).

Next in the arc

Promote the tracks/mixer/switcher off the legacy 'drums' singleton → arr:<idx> + open the drum grid via arrangement selection; then N drums; then the per-arrangement drum_tab build format (with the spec/core FEP); then the region R3b import driver.

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📥 Commits

Reviewing files that changed from the base of the PR and between 736077f and 555d3e1.

📒 Files selected for processing (16)

• CHANGELOG.md
• src/arrangement.js
• src/audio.js
• src/create.js
• src/drum-arrangement.js
• src/file-ops.js
• src/main.js
• src/mixer-panel.js
• src/parts-view.js
• src/stem-tracks.js
• src/track-session.js
• tests/create_drum_materialize.test.js
• tests/drum_arrangement.test.mjs
• tests/mixer_panel.test.mjs
• tests/reorder_part.test.js
• tests/stem_tracks.test.mjs

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• Create PR with unit tests
• Commit unit tests in branch mt-drums-p1

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