v19 - latch note-off detection fixed.

Actual bug in v17/v18 found:
the latch used [route 0] on the original [pitch velocity] list.
That tests PITCH for zero, not VELOCITY, so key-up messages were never
identified as note-offs. Therefore latch ON and OFF had the same duration.

v19 latch ON path now uses [stripnote]:
- note-on (velocity > 0) passes and is stored in [bag]
- note-off (velocity 0) is swallowed
- AUX OFF flushes [bag] as [pitch 0] messages

AUX uses [sel 1], so only button-down toggles.
Built from v17, keeping the confirmed 217956-frame sample-length fix.
samp_mode remains the v17 value (Trigger/2); latch behavior is tested independently.
