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/app.h

@@ -15,7 +15,7 @@ typedef struct { uint8_t used, ch, note; } pad_t;   /* learned pad: ch 0..15 */
 /* Everything the sequencer reads. Core 0 writes it under g_live_lock; core 1 copies it under the same lock. */
 typedef struct {
   sb_state state;
-  int gen, energy, drift, bpm;
+  int gen, keys, energy, drift, bpm;   /* gen = KIT GEN, keys = KEYS GEN */
   int scale, root;                 /* ROOT / SCALE pages: -1 = AUTO (the seed picks) */
   uint8_t on[SB_NT], inv[SB_NT];   /* inv: flipped while the button is held */
   pad_t learn[NKIT];