fix: close review findings on spline EGs — engine, codec, and popup/overlay UI grammar

Live pitch depth, Gate/Spline enable-rule agreement, inert kTrigLength, NaN wire guards, hard-flag-tail corruption no longer wipes the record, RT/cold spline tie-break, retired alt-click, marker-shadow fix, plus new test coverage.

src/shell/instrument/editor_input_deck.cpp

@@ -63,11 +63,19 @@ bool ReaSamplerEditor::mouseDownDeck(const FaceLayout& fl, int x, int y) {
                 applyParamControl(hit.id, 0.0, hit.segment);
                 commitAndReload();
                 break;
-            default:
-                // Parameter-set toggles (play mode / pitch engine / pitch-env + filter enable).
+            default: {
+                // Parameter-set toggles (play mode / pitch engine / pitch-env + filter enable,
+                // and the three env-mode toggles).
                 applyParamControl(hit.id, 0.0, hit.segment);
                 commitAndReload();
+                // Flipping an EG's Staged|Spline toggle makes THAT envelope's overlay active,
+                // so the contour (or the staged shape you just returned to) is what's drawn.
+                // overlayEnvForModeToggle answers kNone for every other toggle this default
+                // case handles, which is why the assignment is conditional.
+                const OverlayEnv modeEnv = overlayEnvForModeToggle(hit.id);
+                if (modeEnv != OverlayEnv::kNone) overlayEnv_ = modeEnv;
                 break;
+            }
         }
         return true;
     }