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>

firmware/test/Makefile

@@ -0,0 +1,25 @@
+# Host build of the engine and the vector checker. No SDK.
+#   make test        build and check against ../../test/vectors
+#   make sanitize    same, with ASan + UBSan
+CC      ?= cc
+CFLAGS  ?= -std=c99 -O2 -Wall -Wextra -Wpedantic -Wshadow -Wconversion -Wno-sign-conversion -Werror
+SRC      = check_vectors.c json.c ../engine/engine.c
+HDR      = json.h ../engine/engine.h
+VECTORS  = ../../test/vectors
+
+check_vectors: $(SRC) $(HDR)
+	$(CC) $(CFLAGS) -o $@ $(SRC)
+
+check_vectors_san: $(SRC) $(HDR)
+	$(CC) $(CFLAGS) -O1 -g -fsanitize=address,undefined -fno-sanitize-recover=all -o $@ $(SRC)
+
+test: check_vectors
+	./check_vectors $(VECTORS)
+
+sanitize: check_vectors_san
+	./check_vectors_san $(VECTORS)
+
+clean:
+	rm -f check_vectors check_vectors_san
+
+.PHONY: test sanitize clean