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/sequencer.c

@@ -125,7 +125,7 @@ static void render_ahead(uint64_t now){
     sb_params p;
     int i;
     snapshot(&L);
-    p.gen = L.gen; p.energy = L.energy; p.drift = L.drift; p.fill = 0; p.scale = L.scale; p.root = L.root;   /* no FILL button: bar-4 fills happen on their own */
+    p.gen = L.gen; p.keys = L.keys; p.energy = L.energy; p.drift = L.drift; p.fill = 0; p.scale = L.scale; p.root = L.root;   /* no FILL button: bar-4 fills happen on their own */
     sb_render_bar(&L.state, &p, bar, &rbar);
     for(i = 0; i < rbar.n; i++){
       enqueue(next_bar_tick + rbar.ev[i].t, 1, &rbar.ev[i]);