instrument: latch the note done at the read-head run-off, fit the migrated fades, and lift the curve dial and overlay selection into pure modules

src/shell/instrument/editor_input_deck.cpp

@@ -51,10 +51,7 @@ bool ReaSamplerEditor::mouseDownDeck(const FaceLayout& fl, int x, int y) {
     const DeckLayout dl = layoutDeck(fl.deckDescs, band.x, band.y, band.width);
     const DeckHit hit = hitTestDeck(dl, x, y);
     if (hit.kind == DeckHitKind::CaptionRadio) {
-        // Exclusive across the three envelope decks, and clicking the active one clears it —
-        // "no envelope shown" is a state the user can get back to, not an error.
-        const OverlayEnv picked = overlayEnvForRadio(hit.id);
-        overlayEnv_ = (overlayEnv_ == picked) ? OverlayEnv::kNone : picked;
+        overlayEnv_ = nextOverlaySelection(overlayEnv_, hit.id);
         invalidate();  // view state only: no parameter write, no reload
         return true;
     }