feat(editor): drums are a selectable part in the arrangement switcher

[ChrisBeWithYou]

Stacked on #336 (which is stacked on #335). Merge order: #335 → #336 → this.

What

Builds on the drums-as-arrangement foundation (#336): the drums arrangement now appears as a "🥁 Drums" option in the part switcher, so you open the drum editor by picking it — exactly like switching to Lead/Rhythm/Bass — instead of a mode button off to the side. Drums become a first-class part in the one place parts are chosen, and the set-up for choosing between multiple drum charts later.

Safe invariant kept

S.currentArr never moves onto the drums arrangement. The drum grid stays a MODE (S.drumEditMode) over the current pitched part — a brand-new currentArr === drums state would have exposed the drums arrangement (empty notes, no tuning) to the pervasive "current arrangement" readers, none of which expect it. So:

• a new editorSwitcherSelect wrapper (the <select> onchange) routes a drums option into drum-edit mode without touching currentArr (mirrors the Tracks 'drums' row / openTrackSessionTarget); a pitched option leaves drum mode and selects it normally;
• editorSelectArrangement stays drums-unaware, so its other callers (undo replay, the Tracks row) are unchanged, and a drums index can never reach it (the isDrumArrangement branch returns first).

Changes

• screen.html: switcher onchange → editorSwitcherSelect.
• main.js: editorSwitcherSelect (new); updateArrangementSelector lists the 🥁-marked drums option and, while drum-edit mode is on, displays it as selected via the new switcherShownIndex pure (currentArr stays pitched); openTrackSessionTarget refreshes the selector.
• drum.js: the 🥁 Edit Drums toggle refreshes the selector too — all three drum entry points keep the dropdown in sync.
• drum-arrangement.js: drumArrangementIndex + switcherShownIndex (pure).

Verification

Runtime-verified (Playwright, isolated host, the one library pack with drums — keys arr + drums): dropdown shows [Keys, 🥁 Drums]; selecting Drums opens the drum grid and shows 🥁 Drums selected; selecting Keys returns the piano roll; the 🥁 Edit Drums toggle also flips the dropdown; no console errors; the pitched view is intact after toggling back (currentArr never landed on drums).

Gates: JS 297/0 · lint 0 err / 3 baseline · routes.py untouched (no pytest). Tests: drum_arrangement.test.mjs +2 (drumArrangementIndex, switcherShownIndex).

Next in the arc

N drums (drop the one-drum block) → the per-arrangement drum_tab build format (needs the feedpak-spec FEP + core loader) → region R3b.

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📥 Commits

Reviewing files that changed from the base of the PR and between 96de8e7 and a723920.

📒 Files selected for processing (7)

• CHANGELOG.md
• screen.html
• src/drum-arrangement.js
• src/drum.js
• src/main.js
• tests/drum_arrangement.test.mjs
• tests/view_switcher.test.mjs

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