feat(player): drum-part picker for multiple drum charts (feedpak 1.17.0)

[ChrisBeWithYou]

The last mile of the multiple-drum-parts feature — stacked on #1020 (base feat/drum-parts-loader). #1020 taught the loader + highway WS to carry several drum charts (song_info.drum_parts, ?drum_part=<id>, a part_id echo on drum_tab); this adds the host-chrome selector that lets a player choose which drummer plays.

What

A Drum part <select> sits beside the arrangement switcher (advanced settings), shown only when a song has 2+ drum charts. Selecting one re-streams that part's tab over the highway WS — exactly like an arrangement switch. Single-drum / no-drum songs hide the row (nothing changes for them).

• static/highway.js — reconnect() gains a drumPart arg → ?drum_part=<id> on the WS URL (mirrors the arrangement param); the song_info handler populates #drum-part-select + shows/hides the row on length > 1 (parallel to the #arr-select block); the drum_tab handler carries msg.part_id onto hwState.drumTab (plugins can read bundle.drumTab.part_id) and reflects it as the picker's selected value (stays honest when the server falls an unknown selection back to the primary).
• static/app.js — changeArrangement() gains an optional drumPart (forwarded, else the current picker selection is preserved so an arrangement switch keeps the chosen part — parts are song-level); new changeDrumPart(id) delegates to it (a part switch is the same re-stream, reusing the transition ceremony). Exported on window.
• static/v3/index.html — the #drum-part-select row (hidden by default).

No plugin change — the drum renderers just draw whatever drum_tab streams. Since the WS reconnect is host-owned, the picker lives in host chrome (like the arrangement switcher), so it works regardless of which drum renderer is active.

Verification

• Runtime-verified, 10/10 (Playwright, the core player, a real 2-drum pack + a no-drum pack): the picker populates with both parts and shows for the multi-drum song; song_info.drum_parts reaches getSongInfo(); the primary is pre-selected; selecting the 2nd part drives highway.reconnect with the id and the WS URL carries ?drum_part=drums-2; the picker then reflects the server's part_id echo; a no-drum song hides the row; no page errors.
• ESLint 0 errors (2 pre-existing max-lines warnings on these already-large files). JS-only; no pytest touched.

⚠️ Stacked on #1020 (feat/drum-parts-loader) — merge that first; CI is main-only until this retargets. And the editor + spec sides of the arc (got-feedBack/feedpak-spec#63, got-feedBack/feedBack-plugin-editor #335→#340) complete the authoring end.

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[ChrisBeWithYou]

Auto-closed, not landed. This PR was closed automatically when its base branch (feat/drum-parts-loader / #1020) was squash-merged to main this morning — the picker never actually landed. Verified: drum-part-select / changeDrumPart / drum_part are all absent from main, even though #1020's loader is in.

Re-landed unchanged as #1028 (cherry-picked onto current main, runtime-verification carried over). Please review there — this thread is dead.

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Reconciliation with #1017 (for review): the two are complementary, not redundant. This picker only renders for songs with 2+ drum charts, so a single drum chart bundled with a melodic instrument (e.g. drums + bass) gets no picker row — and, without #1017's broadened placeholder, no way to reach the drums at all, because #1020 kept the placeholder trigger drum-only. So #1017 fills the gap #1020/#1021 leave open. It merges clean on the new main, but should pick up a CHANGELOG entry + a loader test before it lands.