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>

docs/controls.md

@@ -44,12 +44,13 @@ The Nokia 5110 has 84 × 48 pixels:
 | Page | Edit with the encoder | Push |
 |---|---|---|
 | **SEED** | scroll back through the last 16 seeds (`-1`, `-2` …; `*` = re-rolled) | load the shown seed |
-| **GEN** | EUCLID, SKELETON, SUBDIV, AUTOMATA, AMENIZER | done |
+| **KIT GEN** | EUCLID, SKELETON, SUBDIV, AUTOMATA, AMENIZER | done |
+| **KEYS GEN** | MOTIF, 303, TURING, ARP (see [Concept](concept.md#keys-generators)) | done |
 | **ROOT** | AUTO, C … B. AUTO shows the seed's pick | done |
 | **SCALE** | AUTO, then the 21 scales (see [Concept](concept.md#scales)). AUTO shows the seed's pick | done |
 | **BPM** | tempo | done |
-| **SLOT** | 8 slots for seed + salts + generator (not root or scale) | push = load, long push = save |
-| **ROLL** | KIT or LINE | push re-rolls just that half (a new drum part over the same tune, or the other way round), then goes back to page select |
+| **SLOT** | 8 slots for seed + salts + kit and keys generators (not root or scale) | push = load, long push = save |
+| **ROLL** | KIT or KEYS | push re-rolls just that half, then goes back to page select. KIT: a new drum part over the same keys. KEYS: new chords, scale (on AUTO), motif and patterns over the same drums |
 | **LEARN** | — | push, press a kit track button, hit the pad on the sampler or drum machine. Push again to cancel. |
 
 ## Simulator keys