feat(hx): make HX Stomp MIDI channel configurable

## Summary

The HX Stomp MIDI channel currently defaults to 1 and is not configurable. It should be settable in the pedal definition YAML so users with non-default channel assignments can use `rig apply` correctly.

## Acceptance criteria
- `PedalDefinition` (or the HX Stomp model) accepts an optional `midi_channel` field
- `apply` uses this channel when sending PC messages to the HX Stomp
- Defaults to channel 1 if not specified

Locate the PedalDefinition or HX Stomp model and the apply path that sends PC messages, then trace how pedal definition YAML is parsed. Done means an optional midi_channel is accepted, apply uses it for HX Stomp messages, and unspecified definitions still default to channel 1.

Written by the indexing model from the issue text.