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/shell/instrument/editor_input_chrome.cpp

@@ -1,6 +1,6 @@
 // editor_input_chrome.cpp — the CHROME band's input: the Browse nav, the preview trigger,
-// the preview-velocity knob grab, the channel toggle, and the piano strip's root grab plus
-// its live drag. Windows-only.
+// the preview-velocity knob grab, the loop enable, the channel toggle, and the piano strip's
+// root grab plus its live drag. Windows-only.
 
 #include "shell/instrument/reasampler_editor.h"
 
@@ -73,6 +73,21 @@ bool ReaSamplerEditor::mouseDownChrome(const FaceLayout& fl, int x, int y) {
         invalidate();
         return true;
     }
+    // The loop enable. Inert (not hidden) outside Gate: that refusal comes from the engine and
+    // no click can talk it out of it — unlike the user's own off, which the marks themselves
+    // still offer to reverse.
+    if (loopControlsLive()) {
+        if (contains(cr.loopOff, x, y)) {
+            setLoopEnabled(false);
+            invalidate();
+            return true;
+        }
+        if (contains(cr.loopOn, x, y)) {
+            setLoopEnabled(true);
+            invalidate();
+            return true;
+        }
+    }
     if (contains(cr.chanMono, x, y)) {
         channelMode_ = ChannelMode::Mono;
         processor_->setChannelMode(ChannelMode::Mono);
@@ -146,6 +161,10 @@ ReaSamplerEditor::HoverTarget ReaSamplerEditor::hoverChrome(const FaceLayout& fl
     if (contains(cr.bake, x, y)) return {HoverKind::kBake, -1};
     if (contains(cr.preview, x, y)) return {HoverKind::kPreview, -1};
     if (contains(cr.velCell, x, y)) return {HoverKind::kVelKnob, -1};
+    if (loopControlsLive()) {
+        if (contains(cr.loopOff, x, y)) return {HoverKind::kLoopOff, -1};
+        if (contains(cr.loopOn, x, y)) return {HoverKind::kLoopOn, -1};
+    }
     if (contains(cr.chanMono, x, y)) return {HoverKind::kChanMono, -1};
     if (contains(cr.chanStereo, x, y)) return {HoverKind::kChanStereo, -1};
     if (!cr.rootStrip.empty()) {