Make spline EG drags audible live: held and new notes follow a dragged knot

Contours ride their own seqlock block into a double-buffered engine copy, and the swap step is smoothed. Each drag move publishes contours without a model write; release still commits and reloads.

src/shell/instrument/editor_input_waveform.cpp

@@ -178,6 +178,7 @@ bool ReaSamplerEditor::splineOverlayClick(const OverlayArea& waveArea, int x, in
         // At the ceiling: refused, contour untouched, nothing to roll back. Swallow the click
         // rather than letting it fall through to a marker grab under the cursor.
         if (index < 0) return true;
+        publishContoursLive();  // the new point is heard before the first move, not after it
     }
     drag_ = DragKind::kSplineNode;
     curvePointIndex_ = index;
@@ -188,7 +189,7 @@ bool ReaSamplerEditor::splineOverlayClick(const OverlayArea& waveArea, int x, in
     dragCurveRect_ = waveArea.rect;
     dragStartX_ = x;
     dragStartY_ = y;
-    invalidate();  // live feedback; the commit lands on WM_LBUTTONUP
+    invalidate();  // the model commit lands on WM_LBUTTONUP
     return true;
 }
 
@@ -235,7 +236,8 @@ void ReaSamplerEditor::dragWaveform(const FaceLayout& fl, int x, int y) {
         splineFor(overlayEnv_) = VelocityCurve::resolvePointDrag(
             dragStartCurve_, static_cast<std::size_t>(curvePointIndex_),
             splineOverlayBox(overlay), dx, y - dragStartY_);
-        invalidate();  // live feedback; commit on release
+        publishContoursLive();  // heard now; the model write stays the release's commit
+        invalidate();
         return;
     }