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/ast-parser.js

@@ -328,6 +328,7 @@ function buildBar(node) {
     const bar = {
         type: 'bar',
         id: node.positionals[0] ?? '',
+        isDirty: false,
         stressor: null,
         tempoShape: null,
         tempoLevels: null,