Make bars navigable and editable with _meta export

Navigation:
- SubObjects: bar → voice (with chevron when offsets exist) → offsets
- PaneCP shortView handles voice and offset types

FO pane:
- Bar: editable beats_per_minute, stress_pattern (serialized as
  groups string), upper/lower stress bounds and tempo shape via
  ShapeEditor; sets bar.isDirty on change
- Voice: read-only name, articles, motifs, offset count
- Offset: read-only tick, stem notes and clusters as label strings
  (note re-serialization deferred)

Export:
- exporter.patchScore() now accepts optional bars[] parameter
- Splits rawScoreText by \n---\n; instruments patched in header,
  dirty bar _meta blocks regenerated, voice note lines kept verbatim
- ast-parser buildBar() adds isDirty:false
- PaneCP passes scoreModel.bars to patchScore

static/components/PaneSubObjects.js

@@ -51,7 +51,14 @@ export const PaneSubObjects = {
             }
             if (node.type === 'bar') {
                 return Object.entries(node.voices).map(([name, v]) => ({
-                    kind: 'voice', node: v, label: name,
+                    kind: 'voice', node: v, label: name, hasChildren: v.offsets.length > 0,
+                }));
+            }
+            if (node.type === 'voice') {
+                return node.offsets.map((o, idx) => ({
+                    kind: 'offset', node: o,
+                    label: `tick ${o.tick ?? idx}`,
+                    hasChildren: false,
                 }));
             }
             return [];