Seedbox v0.1 + firmware, generic kit/synth, ROOT/SCALE pages

First commit of the repo. Contents beyond the v0.1 simulator and layout:

- Full Pico 2 W firmware (firmware/app, drivers, engine port, host test).
- BANK page and 404 mode A/B removed; learned pads play exactly what was learned.
- 404/MiniFreak renamed to KIT/SYNTH throughout (jacks, code, sim, docs,
  layout labels; wiring rebuilt).
- ROLL returns to page select after a re-roll; a freshly learned track turns on.
- LCD contrast 66.
- ROOT and SCALE pages: 21 scales, AUTO = the seed picks; global and sticky.

Intended music change: the scale table grew from 8 to 21 and AUTO picks from
all of them, so every seed's scale changed. Vectors regenerated with
npm run vectors, plus new key-override cases. Flash record is now version 2.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/midi-setup.md

@@ -0,0 +1,50 @@
+# MIDI setup
+
+```
+ Seedbox OUT KIT    ──▶ sampler / drum machine MIDI IN
+ Seedbox OUT SYNTH  ──▶ poly synth MIDI IN
+ sampler MIDI OUT   ──▶ Seedbox IN   (only needed to learn pads)
+```
+
+All three Seedbox jacks are 3.5 mm TRS, Type A (tip = pin 5, ring = pin 4, sleeve = pin 2).
+
+| Other end | Cable |
+|---|---|
+| 3.5 mm TRS, Type A (e.g. SP-404MKII) | plain stereo 3.5 mm cable |
+| 5-pin DIN (e.g. MiniFreak) | TRS-to-DIN adapter marked Type A |
+| 3.5 mm TRS, Type B | Type A ↔ B converter |
+
+A Type B adapter on a Type A jack swaps tip and ring: you get no MIDI, but nothing is damaged.
+
+Both OUTs carry MIDI clock (start, 24 PPQN clock, stop) when clock is enabled. Song Position Pointer is not sent.
+
+## Kit (sampler or drum machine)
+
+Pads are learned, not typed in, so any sampler or drum machine that sends a note-on per pad works. Seedbox plays exactly the channel and note it learned. To use another bank or kit, re-learn the pads there.
+
+### Learn a pad
+
+1. Connect the sampler's MIDI OUT to Seedbox IN. In the simulator, choose it as the input port.
+2. LEARN page, push the encoder: the screen says PICK TRACK.
+3. Press a kit track button (KICK … PERC): HIT PAD.
+4. Hit the pad. Seedbox stores its channel and note.
+
+Kit tracks that aren't learned stay silent and show a dot on the screen map.
+
+STOP sends All Sound Off (CC 120) on every learned kit channel. Some samplers ignore All Notes Off (the SP-404MKII does), but they do obey CC 120.
+
+## Synth
+
+- Poly mode, so bass, melody and pad can overlap.
+- Set its MIDI channel to match Seedbox's synth channel (bench setting, default 1).
+- An octave offset is available on the bench.
+- STOP sends CC 120 and CC 123.
+
+## Hardware notes
+
+- The MIDI OUTs follow the MIDI 1.0 3.3 V electrical spec (CA-033):
+  - TX goes through 10 Ω to the tip (pin 5).
+  - 3.3 V goes through 33 Ω to the ring (pin 4).
+  - The sleeve (pin 2) is ground.
+- MIDI IN uses a 6N137 optocoupler powered from 5 V, with its enable pin tied high and its output pulled up to 3.3 V.
+- Details are in [hardware/wiring.md](hardware/wiring.md).