Remove 'r'/'voices=', do not expose explicit voice selection to API.

[dpwe]

Before we added synth=, we used voices= to configure a block of oscs, and to modify several such blocks in parallel. synth still uses the internal voices concept, but now we have removed the possibility to use it externally, which was very limited since you never knew which voice was which. If you want a voice, set up a synth with num_voices=1.

[github-actions]

🎛️ AMY HW CI (AMYboard bench)

Flashed this PR's AMY (LoadTestChord: 6-voice Juno patch=1, one held note every 2 s) onto the physical AMYboard and measured the smoothed render load as the chord grows — back-to-back with the same sketch built at the PR's merge base, so Δ is this PR's own cost.

✅ PASS — the bench ran the test to completion.

notes held	main @ bc13ff9	this PR	Δ
1	1467	1472	+5
2	1664	1660	-4
3	2337	2329	-8
4	2547	2544	-3
5	3276	3275	-1
6	3492	3498	+6

Full chord settled render μs: 3500 (was 3498, Δ +0.1%) (peak 3508, 39 samples)

⬇️ Artifacts: serial log · load trace · report

_{Self-hosted bench (amyboardci). FAIL means only that the test could not run — the load values are informational, with no threshold and no audio compare. See tools/arduino_loadsweep/.}

[bwhitman]

⛓️ tulipcc integration PR opened

This merge was pinned into tulipcc for full-system CI: shorepine/tulipcc#1203

Test it there and merge that PR to move tulipcc onto this AMY.