feat(editor): the drums track is an ordinary arr:<idx> mixer/Tracks channel

[ChrisBeWithYou]

PR2b of the drums-as-arrangement arc — stacked on #337 (base mt-drums-p2), which is stacked on #336 → #335.

What & why

Retire the legacy 'drums' mix-key singleton so the drums arrangement mixes on its own arr:<idx> channel — the same address every pitched part uses. The mixer strip, Tracks mix, band roster, and S.partMix key now all agree. This is the substrate the N-drums work (PR3/PR4) needs: two drum charts can't share one 'drums' slot; each drum arrangement now gets its own strip by index.

Deliberately narrow — holds the #337 invariant: S.currentArr stays pitched and the drum grid stays a mode (S.drumEditMode). Only the mix address moves. The drums arrangement's durable target id stays 'drums' (DRUM_TARGET_ID), so every tracks-row / stemLinks / delete / rename / selection hook that keys on it — and main.js's selectTrackSessionTarget/openTrackSessionTarget 'drums' branches — are untouched.

Also fixes a latent double-strip: #336 materialized the drums arrangement into S.arrangements but left _mixerPartsPure also synthesizing a 'drums' strip from the drum tab — so the mixer showed drums twice (arr:<idx> and 'drums'). The drums strip now comes solely from the arrangement pass.

Changes

• mixer-panel.js — _mixerPartsPure drops the synthesized 'drums' strip (drums ride the arrangement pass as arr:<idx>); new pure _mixerActivePartKeyPure maps drum-edit mode → arr:<drumIdx> (currentArr stays pitched); _mixerClapStatePure takes the drum index; _selectedStripKeyPure + the meter active-part resolve drums via the general id→index path (its id is 'drums').
• track-session.js — the drum target's mixKey → arr:<drumIdx> (id stays 'drums'); DeleteDrumTabCmd saves/restores partMix['arr:<drumIdx>'] (re-derives the index on undo, robust to a shifted pitched count).
• audio.js — _bandPartsPure keys the drum band entry arr:<drumIdx> so the strip and engine agree; the band scheduler routes the type:"drums" arrangement to real GM percussion (_drumKitVoicesInWindow) before the clap-notes path (its own notes are empty), keeping the legacy "Drums"-named pitched-part clap path intact.
• host.js / state.js — partMix key-convention docs.

Verification

• JS 297/0, lint 0 err / 3 baseline, routes.py untouched. Five suites re-pointed to arr:<idx>; added a live-S clap-gate test and a _mixerActivePartKeyPure invariant test (drum mode → arr:<drumIdx>, currentArr ignored; drumIdx < 0 falls back).
• Runtime-verified (Playwright, asdf_asdf.feedpak — the one library pack with drums): the mixer shows a single Drums strip keyed arr:1 (no 'drums' key, no double-strip); mute lights it (aria-pressed, real S.partMix) + status "its guide voice is silent"; solo lights it; the fader scales it (+0.0 → −10.5 dB); drums↔pitched round-trips cleanly.

⚠️ Stacked PR: base is mt-drums-p2 (#337). CI (ci.yml) is main-only, so checks won't run until the base retargets to main after #335→#336→#337 land.

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📥 Commits

Reviewing files that changed from the base of the PR and between f60e5f8 and 9eb1fb3.

📒 Files selected for processing (12)

• CHANGELOG.md
• src/audio.js
• src/host.js
• src/mixer-panel.js
• src/state.js
• src/track-session.js
• tests/drum_arrangement.test.mjs
• tests/drum_delete_undo.test.mjs
• tests/drum_delete_undo_middle.test.mjs
• tests/midi_playback.test.mjs
• tests/mixer_panel.test.mjs
• tests/stem_engine.test.mjs

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• Commit unit tests in branch mt-drums-p2b

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