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_controls.cpp

@@ -142,6 +142,48 @@ instrument::ui::DeckEnableState ReaSamplerEditor::deckEnableState() const {
         play.filterSpline.mode == EnvMode::Spline};
 }
 
+bool ReaSamplerEditor::loopControlsLive() const {
+    return effectivePlayMode(params_.play) == PlayMode::Gate;
+}
+
+instrument::ui::WaveMarks ReaSamplerEditor::waveMarksFor(const SetupMarkers& m) const {
+    using instrument::ui::WaveMark;
+    instrument::ui::WaveMarks w;
+    w.frame[static_cast<int>(WaveMark::kStart)] = m.start;
+    w.frame[static_cast<int>(WaveMark::kLoopStart)] = m.loopStart;
+    w.frame[static_cast<int>(WaveMark::kLoopEnd)] = m.loopEnd;
+    // The crossfade grows LEFT from the seam it closes, which is where it is audible.
+    w.frame[static_cast<int>(WaveMark::kCrossfade)] = m.loopEnd - m.crossfade;
+    // The crossfade mark belongs to an ACTIVE loop: with the enable off there is no seam for it
+    // to sit on, and no length to drag.
+    w.present[static_cast<int>(WaveMark::kStart)] = true;
+    w.present[static_cast<int>(WaveMark::kLoopStart)] = true;
+    w.present[static_cast<int>(WaveMark::kLoopEnd)] = true;
+    w.present[static_cast<int>(WaveMark::kCrossfade)] = m.hasLoop;
+    return w;
+}
+
+instrument::ui::WaveMarks ReaSamplerEditor::grabbableMarks(const SetupMarkers& m) const {
+    using instrument::ui::WaveMark;
+    instrument::ui::WaveMarks w = waveMarksFor(m);
+    if (!loopControlsLive()) {
+        w.present[static_cast<int>(WaveMark::kLoopStart)] = false;
+        w.present[static_cast<int>(WaveMark::kLoopEnd)] = false;
+        w.present[static_cast<int>(WaveMark::kCrossfade)] = false;
+    }
+    return w;
+}
+
+void ReaSamplerEditor::setLoopEnabled(bool on) {
+    const auto frames = static_cast<std::int64_t>(monoPcmFor(selectedId_).size());
+    if (frames <= 0) return;
+    SetupMarkers m = pickedMarkers(frames);
+    if (m.hasLoop == on) return;  // a no-op commit would buy a re-decode for nothing
+    m.hasLoop = on;
+    applyMarkers(m);
+    commitAndReload();
+}
+
 void ReaSamplerEditor::applyDeckKnob(int id, double norm) {
     if (!processor_) return;
     norm = clamp01(norm);