feat(mq_editor): add new partial, undo/redo

- '+ Partial' button (N key): insert 440Hz/max-amp partial at front
- Undo/Redo buttons (Ctrl+Z/Y): JSON snapshot stack, 50 levels
- Hooks in delete, mute, curve edits, amp drag, freq drag

handoff(Gemini): undo/redo + new-partial added to mq_editor

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/tools/mq_editor/viewer.js b/tools/mq_editor/viewer.js
index 723a585..46876c1 100644
--- a/tools/mq_editor/viewer.js
+++ b/tools/mq_editor/viewer.js
@@ -53,6 +53,7 @@ class SpectrogramViewer {
     this.dragState = null; // {pointIndex: 0-3}
     this.onPartialSelect = null; // callback(index)
     this.onRender = null; // callback() called after each render (for synced panels)
+    this.onBeforeChange = null; // callback() called before any mutation (for undo/redo)
 
     // Setup event handlers
     this.setupMouseHandlers();
@@ -569,6 +570,7 @@ class SpectrogramViewer {
             companionOff = { dt: curve.t1 - curve.t0, dv: curve.v1 - curve.v0 };
           else if (ptIdx === 3)
             companionOff = { dt: curve.t2 - curve.t3, dv: curve.v2 - curve.v3 };
+          if (this.onBeforeChange) this.onBeforeChange();
           this.dragState = { pointIndex: ptIdx, companionOff };
           canvas.style.cursor = 'grabbing';
           e.preventDefault();