feat(keyboard): add musical keyboard app

[ArthurGibert]

Summary

• Adds a new app (ID 26) that turns Faderpunk's 16 buttons into a musical keyboard
• Faders set the velocity for each corresponding note (0–4095 → 7-bit MIDI)
• Scale comes from the global quantizer; LEDs update every 150 ms when it changes

Behaviour

LED colours: white keys → white, black/sharp keys → yellow, root/tonic → High brightness, other in-scale → Mid

Two modes (param 4):

• Scale only (default): channels map to the first 16 consecutive in-scale notes starting from Base Note — no dead keys, more reachable notes per press
• Chromatic: channels map to 16 consecutive semitones; out-of-scale keys are still playable but shown at Low brightness

When the scale changes mid-play, held notes are released automatically before the map rebuilds.

Params

#	Name	Default
1	Base Note (i32 0–127)	48 (C3)
2	MIDI Channel	1
3	MIDI Out	USB+DIN
4	Mode (Scale only / Chromatic)	Scale only

Test plan

• Build passes: cargo clippy --bin faderpunk --target thumbv8m.main-none-eabihf -- -D warnings
• Assign app 26 on a channel, set scale to C major — verify 7 white/0 black layout with C highlighted
• Move faders to different positions, press buttons — verify velocity varies
• Switch to D minor, verify root D highlights and scale notes update
• Test Chromatic mode — verify all 16 keys playable, out-of-scale dimmed
• Test on hardware before merging

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