feat(kb): pop piano cell + 8 guitar shapes for thin qualities (task P-62)

Pop piano: 5 progressions x 2 plays (182 total) — triads/inversions
only, one sourced add9 shimmer; ballad rolls vs voice-led pillars,
doo-wop triplets vs one-finger walks, Pachelbel inversion stair,
tonic-drone pads, octave drive. Shapes: dim7/half_dim/aug/maj6/min6
each reach >=3 placeable grips, add9 gains a movable A-shape; dim
honestly left at 2. Audit also surfaced two WRONG pre-existing shapes
(half_dim E-Barre spells {0,3,6,7}; "Open Dadd9" is Dsus2) -> backlog.
Critic PASS (~110 independent checks; all additions pc-spelled in two
keys; strictly additive diff).

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

src/data/kb/index.js

@@ -35,6 +35,7 @@ import gospelPiano from './gospel/piano.js'
 import popMeta from './pop/meta.js'
 import popProgressions from './pop/progressions.js'
 import popGuitar from './pop/guitar.js'
+import popPiano from './pop/piano.js'
 
 export default {
   jazz: {
@@ -85,6 +86,6 @@ export default {
   pop: {
     meta: popMeta,
     progressions: popProgressions,
-    instruments: { guitar: popGuitar },
+    instruments: { guitar: popGuitar, piano: popPiano },
   },
 }