Fix subvariation display, progress bar, and linked-instrument discard

- ast-parser: read for_value= prop for sub-variations (was always null,
  causing fallback to positional "subvariation N" label)
- StatusPoller: calculate progress from currently_rendered_notes /
  notes_in_total; show remaining_time string from status.json
- ShapeEditor/EnvelopeEditor: pass { undo } through onChange so callers
  can revert in-place mutations
- PaneFO: makeChangeHandler stashes undo from info arg; discardEdit
  calls undo() before clearing pendingEdit; depends_on handler provides
  its own undo for the inline node.dependsOn mutation

static/ast-parser.js

@@ -212,7 +212,7 @@ function buildInstrument(node) {
 function buildVariation(node) {
     const v = {
         type: 'variation',
-        dependsOn: node.props.depends_on ?? null,
+        dependsOn: node.props.depends_on ?? node.props.for_value ?? null,
         basicProperties: null,
         labelSpecs: [],
         subvariations: [],