Split kit and keys; KEYS GEN with MOTIF, 303, TURING, ARP

- genome(seed, salts) now has three parts with their own RNG streams:
  shared (steps, form, swing, arc, layback, fill odds, phase offset),
  kit (density, frame, accent cycle, free snare) and keys (scale/root,
  progression, motif, hocket, bass tone, note lengths, range). ROLL KEYS
  now renews the harmony too; ROLL KIT leaves the keys alone.
- Keys generators share one signature and can fix note lengths:
  MOTIF (the old line), 303 (acid bass, answers in the rests, drone or
  silent pad), TURING (8/16-bit shift register, DRIFT flips bits, 16-bar
  window), ARP (triad/7th arpeggio, roots on kicks).
- Pages: KIT GEN, KEYS GEN; ROLL shows KIT / KEYS; slots keep both gens.
- Flash record v3 (old settings ignored). Bring-up golden checksums updated.
- C port matches; host test and ASan/UBSan pass.

Intended music change: every seed sounds different (genome split, new
generators). Vectors regenerated with npm run vectors; the case grid now
crosses all kit and keys generators, with TURING runs past its window.

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

firmware/app/storage.h

@@ -1,4 +1,4 @@
-/* Settings in the last flash sector: slots, learned pads, BPM, generator, root, scale, synth channel and octave,
+/* Settings in the last flash sector: slots, learned pads, BPM, kit and keys generators, root, scale, synth channel and octave,
    plus the current seed and track on/off (as app.js keeps them). Erasing a sector stalls both cores for tens of
    milliseconds, so main.c only saves while stopped. */
 #ifndef SEEDBOX_STORAGE_H
@@ -6,7 +6,7 @@
 
 #include "app.h"
 
-typedef struct { uint8_t used, gen; uint32_t seed; uint32_t salts[2]; } slot_t;
+typedef struct { uint8_t used, gen, keys; uint32_t seed; uint32_t salts[2]; } slot_t;
 
 void storage_defaults(live_t *L, slot_t *slots);
 int  storage_load(live_t *L, slot_t *slots);         /* 1 if a valid record was found */