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/editor_input_browse.cpp

@@ -131,9 +131,12 @@ void ReaSamplerEditor::onMouseWheel(int delta) {
 
 void ReaSamplerEditor::onSearchChar(unsigned int ch) {
     // The curve popup: Esc dismisses (checked first — the popup is modal over the face, and
-    // the Browse search cannot hold focus under it).
+    // the Browse search cannot hold focus under it). Esc reaches here unconditionally
+    // (editor_platform's WM_CHAR routing), including mid-drag on a curve node — closeCurvePopup
+    // cancels that drag too, or a subsequent WM_MOUSEMOVE would resolve editedCurve() with the
+    // popup already closed.
     if (curvePopup_ != CurveTarget::kNone && ch == 27) {
-        curvePopup_ = CurveTarget::kNone;
+        closeCurvePopup();
         invalidate();
         return;
     }