v17 latch test, rebuilt from confirmed-good v13.

Important fixes versus v16:
- AUX is edge-filtered with [sel 1], so button release (0) cannot toggle again.
- latch state is stored explicitly in [f 0] and written back after every toggle.
- OFF path is the exact normal v13 note stream.
- ON path passes/stores note-ons and swallows note-offs.
- switching OFF flushes stored notes.
- sample length remains fixed at 217956 frames.
No pitch/voice changes.
