feat(mq_editor): drag anchor points P0/P3 with their companion handles

Moving P0 translates P1 rigidly (preserving relative offset).
Moving P3 translates P2 rigidly. Handles P1/P2 remain independently
draggable.

handoff(Claude): bezier coupled anchor drag done

1 files changed, 13 insertions, 2 deletions

diff --git a/tools/mq_editor/editor.js b/tools/mq_editor/editor.js
index 301dcc5..868d3d5 100644
--- a/tools/mq_editor/editor.js
+++ b/tools/mq_editor/editor.js
@@ -23,7 +23,8 @@ class PartialEditor {
 
     // Private state
     this._selectedIndex  = -1;
-    this._dragPointIndex = -1;
+    this._dragPointIndex    = -1;
+    this._dragCompanionOff  = null; // {dt, dv} offset of companion handle relative to anchor
     this._amp = { tMin: 0, tMax: 1, ampTop: 1 };
 
     this._setupButtons();
@@ -489,7 +490,12 @@ class PartialEditor {
       const curve = partial.ampCurve;
       for (let i = 0; i < 4; ++i) {
         if (Math.hypot(this._tToX(curve['t' + i]) - x, this._ampToY(curve['v' + i]) - y) <= 8) {
-          this._dragPointIndex = i;
+          this._dragPointIndex   = i;
+          this._dragCompanionOff = null;
+          if (i === 0)
+            this._dragCompanionOff = { dt: curve.t1 - curve.t0, dv: curve.v1 - curve.v0 };
+          else if (i === 3)
+            this._dragCompanionOff = { dt: curve.t2 - curve.t3, dv: curve.v2 - curve.v3 };
           canvas.style.cursor  = 'grabbing';
           e.preventDefault();
           return;
@@ -505,6 +511,11 @@ class PartialEditor {
         const i     = this._dragPointIndex;
         curve['t' + i] = Math.max(0, Math.min(this.viewer ? this.viewer.t_max : 1e6, this._xToT(x)));
         curve['v' + i] = Math.max(0, this._yToAmp(y));
+        if (this._dragCompanionOff) {
+          const off = this._dragCompanionOff;
+          if (i === 0) { curve.t1 = curve.t0 + off.dt; curve.v1 = curve.v0 + off.dv; }
+          else          { curve.t2 = curve.t3 + off.dt; curve.v2 = curve.v3 + off.dv; }
+        }
         this._renderAmpEditor();
         if (this.viewer) this.viewer.render();
         e.preventDefault();