fix(voicings): correct movable-shape base-fret indexing in getGuitarVoicings (task L-23)

strIdx = rootStr-1 read OPEN [s6..s1] backwards: every rootStr-5 shape
sat 2 frets low, every rootStr-4 shape 5 frets low (A-shape C major
labeled fret 1 instead of 3) in ChordDetailModal/ExplorePanel/
CurrentJamPanel; rootStr-6 was accidentally correct (same pc both ends).
Now uses the shared 6-rootStr convention. Also: playChord docstring
octave example corrected. Critic PASS (6 independent fret checks +
252-entry pre/post sweep, structure unchanged).

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

src/lib/voicings.js

@@ -504,8 +504,11 @@ export function getGuitarVoicings(chordName) {
       continue
     }
 
-    // barre shape: compute rootFret on rootStr
-    const strIdx = shape.rootStr - 1  // 0=s6 … 5=s1
+    // barre shape: compute rootFret on rootStr.
+    // OPEN is ordered [s6 … s1] and rootStr is 1-indexed with 6 = low E,
+    // so string N lives at index 6 - N (same convention as ChordDiagram.jsx
+    // and chordAudio.guitarShapeToNotes).
+    const strIdx = 6 - shape.rootStr  // 0=s6 … 5=s1
     const openPc = OPEN[strIdx]
     let rootFret = (rootPc - openPc + 12) % 12