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.cpp

@@ -121,7 +121,7 @@ void ReaSamplerEditor::onMouseUp(int x, int y) {
                          y < curveRect.y - kCurveDragOffMargin ||
                          y > curveRect.bottom() + kCurveDragOffMargin;
         if (off) {
-            params_.velocityCurve.deletePoint(static_cast<std::size_t>(curveIdx));
+            editedCurve().deletePoint(static_cast<std::size_t>(curveIdx));
             hover_ = HoverTarget{};  // stale kCurveNode index would light a shifted node
         }
     }
@@ -139,7 +139,7 @@ void ReaSamplerEditor::resolveHover(int x, int y) {
     HoverTarget h;  // kNone by default
     if (view_ == View::kBrowse) {
         h = hoverBrowse(w, hgt, x, y);
-    } else if (curvePopupOpen_) {  // modal over the face
+    } else if (curvePopup_ != CurveTarget::kNone) {  // modal over the face
         h = hoverCurvePopup(w, hgt, x, y);
     } else {
         const FaceLayout fl = faceLayout(w, hgt);