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/PaneFO.js

@@ -39,11 +39,11 @@ export const PaneFO = {
 
         // Returns a change handler that intercepts the first edit to a linked
         // instrument and shows the embed-or-discard modal before applying it.
+        // `info` may carry { undo } forwarded from ShapeEditor / EnvelopeEditor.
         function makeChangeHandler(instr, apply) {
-            return () => {
+            return (info) => {
                 if (instr.isLinked && !instr.isDirty) {
-                    // Stash the apply callback and show the modal.
-                    pendingEdit.value = { instr, apply };
+                    pendingEdit.value = { instr, apply, undo: info?.undo };
                 } else {
                     apply();
                     instr.isDirty = true;
@@ -62,6 +62,7 @@ export const PaneFO = {
         }
 
         function discardEdit() {
+            pendingEdit.value?.undo?.();
             pendingEdit.value = null;
         }
 
@@ -104,8 +105,13 @@ export const PaneFO = {
                 children.push(
                     h('h4', { style: 'margin:0 0 0.5rem' }, 'Variation'),
                     h(ObjectExtended, { fields: variationFields(node), onChange: ({ key, value }) => {
-                        if (key === 'depends_on') node.dependsOn = value;
-                        onChange();
+                        if (key === 'depends_on') {
+                            const old = node.dependsOn;
+                            node.dependsOn = value;
+                            onChange({ undo: () => { node.dependsOn = old; } });
+                        } else {
+                            onChange({});
+                        }
                     }}),
                     node.basicProperties
                         ? h(EnvelopeEditor, { basicProperties: node.basicProperties, onChange })