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.

tests/test_component_state_io.cpp

@@ -1089,6 +1089,27 @@ static void testLoopSpanAndCrossfadeRoundTrip() {
     CHECK(out.params.rootOverride && *out.params.rootOverride == 55);
 }
 
+// The loop ENABLE is `hasLoop`, and the block writes start/end unconditionally — so the wire
+// already carries "off, with a span remembered." Nothing about the format changes to make the
+// enable user-owned; this pins that the off state and its retained span both survive a reload,
+// because an off that came back as a re-parked default would be a delete button, not a toggle.
+static void testAnOffLoopRoundTripsWithItsSpanAndCrossfadeRetained() {
+    ComponentState in;
+    in.selectionId = "pad";
+    SampleLoop lp;
+    lp.hasLoop = false;
+    lp.start = 4096;
+    lp.end = 65536;
+    in.params.loopOverride = lp;
+    in.params.loopCrossfadeFrames = 1024;
+
+    const ComponentState out = deserializeComponentState(serializeComponentState(in), 48000.0);
+    CHECK(out.params.loopOverride && !out.params.loopOverride->hasLoop);
+    CHECK(out.params.loopOverride && out.params.loopOverride->start == 4096);
+    CHECK(out.params.loopOverride && out.params.loopOverride->end == 65536);
+    CHECK(out.params.loopCrossfadeFrames == 1024);
+}
+
 // A negative fade cannot mean anything and would only reach resolveLoop's clamp; refusing it
 // at the wire keeps the parameter set the editor reads back sane.
 static void testNegativeCrossfadeOnTheWireLiftsToZero() {
@@ -2039,6 +2060,7 @@ int main() {
     testDefaultStateRoundTripsToDefaults();
     testEnvelopePrefixBytesFrozen();
     testLoopSpanAndCrossfadeRoundTrip();
+    testAnOffLoopRoundTripsWithItsSpanAndCrossfadeRetained();
     testNegativeCrossfadeOnTheWireLiftsToZero();
     testPriorPayloadVersionsLiftToAHardSeam();
     testLimiterEnableRoundTripsAndV14LiftsToBypassedWithItsHoldIntact();