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/StatusPoller.js

@@ -1,35 +1,20 @@
-import { h, ref, onMounted, onUnmounted } from 'vue';
+import { h, onMounted, onUnmounted, ref } from 'vue';
 import { fetchStatus } from '../api.js';
 
 export const StatusPoller = {
     props: ['store'],
     setup(props) {
         const timer = ref(null);
-        const eta = ref(null);
-
-        function nextInterval(status) {
-            if (!status) return 2000;
-            const pct = status.progress ?? 0;
-            // At 0–20%: poll at centile-of-ETA intervals
-            if (eta.value && pct > 0 && pct <= 20) {
-                return Math.max(500, (eta.value * 10) | 0);
-            }
-            return 2000;
-        }
 
         async function poll() {
             try {
                 const status = await fetchStatus(props.store.credentials);
-                if (status.eta) eta.value = status.eta;
                 props.store.synthesisStatus = status;
-                if (status.frozen) {
-                    // done — stop polling
-                    return;
-                }
+                if (status.frozen) return;
             } catch (_) {
-                // transient error — keep polling
+                // transient — keep polling
             }
-            timer.value = setTimeout(poll, nextInterval(props.store.synthesisStatus));
+            timer.value = setTimeout(poll, 2000);
         }
 
         onMounted(() => { poll(); });
@@ -39,10 +24,16 @@ export const StatusPoller = {
             const s = props.store.synthesisStatus;
             if (!s) return h('div', { class: 'se-status' }, 'Polling…');
 
-            const pct = s.progress ?? 0;
-            const label = s.frozen
-                ? (s.error ? `Error: ${s.error}` : 'Done')
-                : `${s.state ?? 'Running'} ${pct}%`;
+            const total = s.notes_in_total ?? 0;
+            const done  = s.currently_rendered_notes ?? 0;
+            const pct   = total > 0 ? Math.min(100, Math.round(done / total * 100)) : 0;
+
+            let label;
+            if (s.frozen) {
+                label = s.errors ? `Error: ${s.errors}` : 'Done';
+            } else {
+                label = s.remaining_time ?? `Synthesizing… ${pct}%`;
+            }
 
             return h('div', { class: 'se-status' }, [
                 h('span', null, label),