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/components/EnvelopeEditor.js

@@ -24,12 +24,9 @@ export const EnvelopeEditor = {
     setup(props) {
         function toggle(section) {
             const bp = props.basicProperties;
-            if (bp[section]) {
-                bp[section] = null;
-            } else {
-                bp[section] = defaultShape(section);
-            }
-            props.onChange?.();
+            const old = bp[section];
+            bp[section] = old ? null : defaultShape(section);
+            props.onChange?.({ undo: () => { bp[section] = old; } });
         }
 
         return () => {
@@ -57,7 +54,7 @@ export const EnvelopeEditor = {
                         ? h('div', { class: disabled ? 'se-envelope-disabled' : null }, [
                             h(ShapeEditor, {
                                 shape: bp[key],
-                                onChange: props.onChange,
+                                onChange: info => props.onChange?.(info),
                             }),
                           ])
                         : null,