instrument: spline EGs — hard points on the one shared spline, a drawn contour per envelope beside its staged state, payload v13

src/core/instrument/ui/knob_deck.h

@@ -64,6 +64,11 @@ struct DeckGroupDesc {
     int captionWidth = 60;
     DeckRadioDesc captionRadio;    // the caption row's far corner; id -1 = none
     DeckToggleDesc captionToggle;  // caption row, left of the radio; id -1 = none
+    // A second caption toggle, placed immediately left of the first (or in its place when the
+    // first is absent). Exists because a group whose knob row is wider than its caption row has
+    // caption slack a toggle can occupy for free — a rowToggle would widen the GROUP, and the
+    // deck has six pixels of headroom on its first row at the editor's floor width.
+    DeckToggleDesc captionToggle2;
     std::vector<int> cellIds;      // knob cells; -1 = blank reserve
     DeckToggleDesc rowToggle;      // in the knob row after the cells; id -1 = none
 };
@@ -95,6 +100,7 @@ struct DeckGroupLayout {
     Rect caption;                   // caption text rect (left part of the caption row)
     DeckRadioLayout captionRadio;   // id -1 when absent (rect empty)
     DeckToggleLayout captionToggle; // id -1 when absent (rects empty)
+    DeckToggleLayout captionToggle2;
     std::vector<DeckCellLayout> cells;
     DeckToggleLayout rowToggle;     // id -1 when absent
 };