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_session.cpp

@@ -72,7 +72,7 @@ void ReaSamplerEditor::refreshFromBank() {
     monoTrigger_ = processor_->monoTrigger();
     // A refresh that emptied the selection closes the curve popup — an open-but-invisible
     // modal would otherwise swallow clicks on the empty state.
-    if (selectedId_.empty()) curvePopupOpen_ = false;
+    if (selectedId_.empty()) curvePopup_ = CurveTarget::kNone;
     // Drop a filter that names a bank no longer present.
     if (!activeFilterBankId_.empty()) {
         bool found = false;