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_deck.cpp

@@ -28,8 +28,10 @@ bool ReaSamplerEditor::deckKnobDisabled(int id) const {
         case ParamControl::kFilterQ:
         case ParamControl::kFilterDrive:
         case ParamControl::kFilterModAmt:
-        case ParamControl::kFilterVel:
         case ParamControl::kFilterKeyTrack:
+        // The filter's velocity curve sits in the VELOCITY group but is a filter parameter:
+        // it goes inert with every other one, so no surface can reach a param the knobs can't.
+        case ParamControl::kFilterVelCurve:
         case ParamControl::kFilterEnvAttack:
         case ParamControl::kFilterEnvHold:
         case ParamControl::kFilterEnvDecay:
@@ -96,6 +98,13 @@ bool ReaSamplerEditor::mouseDownDeck(const FaceLayout& fl, int x, int y) {
     if (hit.kind == DeckHitKind::Knob) {
         // Knobs of a disabled group are drawn but inert.
         if (deckKnobDisabled(hit.id)) return true;
+        // A VELOCITY cell summons the popup editor instead of starting a knob drag.
+        const CurveTarget curveCell = curveTargetFor(hit.id);
+        if (curveCell != CurveTarget::kNone) {
+            curvePopup_ = curveCell;
+            invalidate();
+            return true;
+        }
         // A grab on the inner disc drags the CURVE control instead, but only where the stage
         // is sloped; on a Hold or Sustain cell the inner region is just more of the knob.
         const ParamControl curve = curveParamFor(static_cast<ParamControl>(hit.id));