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/engine/engine.h

@@ -17,6 +17,7 @@
 enum { SB_T_KICK, SB_T_KICK2, SB_T_SNARE, SB_T_GHOST, SB_T_HAT, SB_T_HAT16, SB_T_PERC,
        SB_T_BASS, SB_T_MELA, SB_T_MELB, SB_T_PAD };
 enum { SB_GEN_EUCLID, SB_GEN_SKELETON, SB_GEN_SUBDIV, SB_GEN_AUTOMATA, SB_GEN_AMENIZER, SB_NGEN };
+enum { SB_KEYS_MOTIF, SB_KEYS_303, SB_KEYS_TURING, SB_KEYS_ARP, SB_NKEYS };
 
 /* hash and RNG */
 uint32_t sb_mix(uint32_t a, uint32_t b);
@@ -38,11 +39,12 @@ typedef struct {
   int prog[4];
   int swing, arc, motifLen, accentCycle, layback, frame, freeK, freeRot;
 } sb_genome;
-void sb_genome_make(uint32_t seed, sb_genome *g);
+typedef struct { uint32_t seed; uint32_t salts[2]; } sb_state;
+/* shared part from the seed; kit and keys parts also take their salt (ROLL KIT / ROLL KEYS) */
+void sb_genome_make(const sb_state *st, sb_genome *g);
 const char *sb_form_name(int form_idx);
 
-typedef struct { uint32_t seed; uint32_t salts[2]; } sb_state;
-typedef struct { int gen, energy, drift, fill, scale, root; } sb_params;   /* scale, root: -1 = AUTO (the seed picks) */
+typedef struct { int gen, keys, energy, drift, fill, scale, root; } sb_params;   /* scale, root: -1 = AUTO (the seed picks) */
 
 /* one output row: [t, lane, v, vel, len, tr] */
 typedef struct { int16_t t, len; uint8_t lane, v, vel, tr; } sb_event;
@@ -56,7 +58,8 @@ typedef struct {
 
 /* ROOT / SCALE pages: overrides the genome's scale and root when they're >= 0 (app.js applyKey). */
 void sb_apply_key(sb_genome *g, const sb_params *p);
-const char *sb_scale_name(int scale);   /* screen name, at most 10 characters */
+const char *sb_scale_name(int scale);
+const char *sb_keys_name(int keys);     /* MOTIF, 303, TURING, ARP */   /* screen name, at most 10 characters */
 
 void sb_render_bar(const sb_state *st, const sb_params *p, int bar, sb_bar *out);