deck: filter mod moves to FILTER ENV, cell runs centre in their reserves, two-segment toggles become single buttons, deck focuses its overlay

src/core/instrument/ui/sample_chrome.h

@@ -33,13 +33,12 @@ struct ChromeRects {
     Rect velCell;     // preview-velocity knob cell (knob + label band)
     Rect velKnob;
     Rect velLabel;
-    // The sustain loop's enable. Immediately left of the channel toggle because it is the same
-    // class of control — a playback mode of the loaded capture — and because the run is
-    // right-anchored, so the title slot absorbs its width and the editor's floor does not move.
-    Rect loopOff;
-    Rect loopOn;
-    Rect chanMono;
-    Rect chanStereo;
+    // The sustain loop's enable, ONE button. Immediately left of the channel selector because
+    // it is the same class of control — a playback mode of the loaded capture — and because the
+    // run is right-anchored, so the title slot absorbs its width and the editor's floor does
+    // not move.
+    Rect loop;
+    Rect channel;     // the mono/stereo MODE selector: its label reads the current mode
     Rect navBrowse;
     Rect controls;    // full-width second row
     Rect rootStrip;   // the piano strip: the whole row, inset only by the shared band pad