feat(mq_editor): movable inner bezier control points + clamp() refactor

- P1/P2 in amp editor now draggable horizontally; t0<t1<t2<t3 enforced
- Add clamp() to utils.js; replace all Math.max/min clamping patterns
- Cursor hints: move for P1/P2, ns-resize for P0/P3
- Remove test_fft.html
- Docs: Delete key, style.css/utils.js in architecture, bezier editor section

handoff(Claude): inner control points done, clamp() adopted everywhere

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

1 files changed, 13 insertions, 6 deletions

diff --git a/tools/mq_editor/editor.js b/tools/mq_editor/editor.js
index a7d0879..6ea6d73 100644
--- a/tools/mq_editor/editor.js
+++ b/tools/mq_editor/editor.js
@@ -322,10 +322,10 @@ class PartialEditor {
       if (!dragging) return;
       const dx = e.clientX - startX;
       const half = slider.offsetWidth / 2;
-      const clamped = Math.max(-half, Math.min(half, dx));
+      const clamped = clamp(dx, -half, half);
       thumb.style.transition = 'none';
       thumb.style.left = `calc(50% - 3px + ${clamped}px)`;
-      const newVal = Math.max(min, Math.min(max, startVal + dx * sensitivity));
+      const newVal = clamp(startVal + dx * sensitivity, min, max);
       inp.value = newVal.toFixed(decimals);
       onUpdate(newVal);
     };
@@ -496,7 +496,7 @@ class PartialEditor {
         if (Math.hypot(this._tToX(curve['t' + i]) - x, this._ampToY(curve['a' + i]) - y) <= 8) {
           if (this.onBeforeChange) this.onBeforeChange();
           this._dragPointIndex = i;
-          canvas.style.cursor  = 'grabbing';
+          canvas.style.cursor  = (i === 1 || i === 2) ? 'move' : 'ns-resize';
           e.preventDefault();
           return;
         }
@@ -510,6 +510,12 @@ class PartialEditor {
         const curve = this.partials[this._selectedIndex].freqCurve;
         const i     = this._dragPointIndex;
         curve['a' + i] = Math.max(0, this._yToAmp(y));
+        // Inner control points are also horizontally draggable
+        if (i === 1) {
+          curve.t1 = clamp(this._xToT(x), curve.t0 + 1e-4, curve.t2 - 1e-4);
+        } else if (i === 2) {
+          curve.t2 = clamp(this._xToT(x), curve.t1 + 1e-4, curve.t3 - 1e-4);
+        }
         this._renderAmpEditor();
         if (this.viewer) this.viewer.render();
         e.preventDefault();
@@ -520,13 +526,14 @@ class PartialEditor {
       if (this._selectedIndex >= 0 && this.partials) {
         const curve = this.partials[this._selectedIndex]?.freqCurve;
         if (curve) {
-          let near = false;
+          let cursor = 'crosshair';
           for (let i = 0; i < 4; ++i) {
             if (Math.hypot(this._tToX(curve['t' + i]) - x, this._ampToY(curve['a' + i]) - y) <= 8) {
-              near = true; break;
+              cursor = (i === 1 || i === 2) ? 'move' : 'ns-resize';
+              break;
             }
           }
-          canvas.style.cursor = near ? 'grab' : 'crosshair';
+          canvas.style.cursor = cursor;
         }
       }
     });