instrument: one VELOCITY deck for all three velocity curves, bipolar and off by default for pitch and filter

Payload v12 appends the new velocity->pitch curve and folds the retired filter velAmount into its now-bipolar curve, so pre-v12 projects reopen sounding identical. Preview button takes a drawn play triangle.

src/shell/instrument/editor_input_browse.cpp

@@ -132,8 +132,8 @@ 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).
-    if (curvePopupOpen_ && ch == 27) {
-        curvePopupOpen_ = false;
+    if (curvePopup_ != CurveTarget::kNone && ch == 27) {
+        curvePopup_ = CurveTarget::kNone;
         invalidate();
         return;
     }