feat(kb): neo-soul/rnb piano cell — 7 progressions x 2 authored plays (task P-30)

Signature comp voicings over the computed fallback: m9 spreads, So What
quartal planing, Stevie 9sus cadence, m6/9 borrowed iv, 13sus, one 7#9
per loop; 3 comping cells + 5 improv rows. KB now 154 plays. Critic
PASS first try (independent arithmetic on 11 prose claims, spans <=11,
all 12 keys render in the 36-key window).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/data/kb/index.js

@@ -25,6 +25,7 @@ import countryGuitar from './country/guitar.js'
 import rnbMeta from './rnb/meta.js'
 import rnbProgressions from './rnb/progressions.js'
 import rnbGuitar from './rnb/guitar.js'
+import rnbPiano from './rnb/piano.js'
 import gospelMeta from './gospel/meta.js'
 import gospelProgressions from './gospel/progressions.js'
 import gospelGuitar from './gospel/guitar.js'
@@ -72,7 +73,7 @@ export default {
   rnb: {
     meta: rnbMeta,
     progressions: rnbProgressions,
-    instruments: { guitar: rnbGuitar },
+    instruments: { guitar: rnbGuitar, piano: rnbPiano },
   },
   gospel: {
     meta: gospelMeta,