Add test for midi_note_cmd; note_source now reports MIDI channel.

[dpwe]

I wanted to update the routing of events through midi_mappings to allow additional, non-MIDI options like pan, so I started with a test. But before that I realized I didn't have a test of conventional midi_note_cmd operation, so I added one, then found that it didn't work. So I got into fixing routing through the MIDI layer, particularly for SYNTH_FLAGS_NOTES_VIA_MIDI, which is problematic as noted in #782.

This change fixes the problem of possible loops in midi_mappings processing by setting note_source to the MIDI channel on which the event was received for events processed by midi_mappings. If those mapped events are again considered for mapping (at patches.c:792), they are skipped because note_source indicates they have already been processed. By using the MIDI channel, it's possible to stack midi_mappings on different synths (but does not protect against loops where synth X forwards to synth Y, then synth Y sends back to synth X).

[bwhitman]

🎛️ AMYboard HW CI

Dispatched this PR's amy SHA to the tulipcc bench — it builds the AMYboard firmware with your change and runs it on the physical board (audio spectral-compared to a committed reference). Results post here in a few minutes.

[bwhitman]

🎛️ HW CI (physical bench)

Built from this AMY PR, pinned into the tulipcc amy submodule.

AMYboard (USB-MIDI + AMY zP → audio; built-in tones + AMYboard World sketches acid/house/woodpiano over the SysEx control API): ❌ FAIL — a check did not match, or the run errored. See the log/artifacts.

Tulip (TULIP4_R11; serial-REPL audio + WiFi screenshot): ✅ PASS — flashed this PR’s firmware; all checks matched the references.

⬇️ Artifacts: recordings · screenshot · serial logs · run logs

_{Self-hosted bench. Audio spectral-compared to ref/hwci_basic.wav, the AMYboard World sketch refs (ref/{acid_generator,house_generator,woodpiano}.wav) + ref/tulip_basic.wav; Tulip screenshot pixel-compared to ref/tulip_screenshot.png. Both analog outs share one capture card, so the tests run sequentially.}

[bwhitman]

⛓️ tulipcc integration PR opened

This merge was pinned into tulipcc for full-system CI: shorepine/tulipcc#1085

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