kb: add rock guitar pack; schema gains declared omit3 (power chords)

5 progressions (Mixolydian vamp, I-IV-V, minor descent, axis, Dorian
riff cell) x 2 plays: open-chord Malcolm hits, second-guitar triads,
Keith sus figure, power-chord chug, Police add9 arpeggios, thumb-over
embellishments. omit3 is a declared omission like rootless - the
pitch-class check still applies. Validator and build green.

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

src/data/kb/SCHEMA.md

@@ -85,6 +85,9 @@ export default {
         // open shapes instead use: frets: [...absolute], onlyRoot: <pc 0-11>
       },
       extensions: ['9'],   // declared color tones beyond the quality (validator allows only these)
+      // declared omissions (honest data, surfaced by the UI):
+      // rootless: true — shape omits the root (e.g. guide-tone grips)
+      // omit3: true    — shape omits the 3rd (e.g. power chords; works over major or minor)
       note: 'root–♭7–♭3',
     },
     // …