fix: close review findings on the velocity-curve deck and bipolar curves

Cancels the curve-node drag whenever the popup closes so Esc mid-drag can't alias the amp curve; generalizes CurveTarget routing to one switch; fixes stale/overstated comments; clamps a pre-v12 depth fold; adds deck-inertness and filter-fold test coverage.

src/shell/instrument/reasampler_editor.h

@@ -150,7 +150,7 @@ private:
 
     // --- Band painters (one TU each, mirroring the input side) ---
     // Chrome: title band + Browse nav + the control row (root strip, preview, velocity knob,
-    // curve button, channel toggle).
+    // channel toggle).
     void paintChrome(LICE_IBitmap* bmp, const FaceLayout& fl, bool empty);
     // The hovered piano key's note-name chip. Drawn after every band — it overhangs the
     // chrome into whatever is below it.
@@ -459,11 +459,22 @@ private:
     // delta from this anchor, so a grab never jumps the value.
     double dragKnobStartValue_ = 0.0;
 
-    // Which velocity curve the popup is editing; kNone = closed. Never persisted.
+    // Which velocity curve the popup is editing; kNone = closed. Never persisted. Every writer
+    // of kNone must also cancel a live curve-node drag (closeCurvePopup does both) — an Esc
+    // mid-drag that closed the popup without cancelling the drag used to leave editedCurve()'s
+    // mutable overload aliasing the amp curve underneath an in-flight pitch/filter drag.
     CurveTarget curvePopup_ = CurveTarget::kNone;
-    // The parameter-set curve `curvePopup_` names. Both overloads exist because every edit
-    // path needs the mutable one and paint needs the const one; routing through this ONE
-    // switch is what keeps the three curves on a single popup/draw/hit-test code path.
+    void closeCurvePopup();
+
+    // THE one switch from a CurveTarget to the parameter-set curve it names — paint (button
+    // thumbnails, for every target) and edit (editedCurve, for curvePopup_ specifically) both
+    // route through it, so a fourth curve or a moved field is a one-place edit.
+    const VelocityCurve& curveFor(CurveTarget target) const;
+    VelocityCurve& curveFor(CurveTarget target);
+
+    // The curve curvePopup_ names — curveFor(curvePopup_), typed as its own pair because every
+    // edit path needs the mutable overload and paint needs the const one. The mutable overload
+    // refuses kNone (a closed popup has nothing open to edit) rather than aliasing amp.
     VelocityCurve& editedCurve();
     const VelocityCurve& editedCurve() const;