Loop: an explicit enable, four named marks with grabbable caps, and the crossfade painted where it is actually heard

hasLoop becomes user-owned with the gestures as shortcuts onto it; no format change. START uses overlay/trace, not accent/primary, which is the waveform's own fill.

src/core/instrument/ui/sample_chrome.h

@@ -2,7 +2,8 @@
 // sample_chrome.h — interior geometry of the Sample face's CHROME band: the toolbar row
 // (title + the whole right-anchored control run + Browse) over the strip row, which the
 // piano strip has to itself. Reads the band rect the allocator hands it (sample_bands) and
-// never allocates vertical space of its own.
+// never allocates vertical space of its own. The run is right-anchored and the title takes
+// the remainder, so a member added to the run costs the title, never the window's floor.
 
 #include "core/instrument/ui/editor_geometry.h"  // Rect
 
@@ -32,6 +33,11 @@ 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;
     Rect navBrowse;