loop: fix the crossfade seam's residual discontinuity, plus six review minors

Normalizes crossfadeWeight over crossfade-1 so the last rendered frame lands at exactly the incoming tap instead of a residual step; corrects the CLAUDE.md invariant and seam test to match. Shares lerpSource/crossfadedSource/maxCrossfade, fixes stale docs/constants, and clears crossfade on the loop-OFF gesture.

tests/test_waveform_view.cpp

@@ -361,6 +361,28 @@ static void testMarkerHandleOnDegenerateAreas() {
     CHECK(markerHandleRect(overlayOf(thin), 1000, 500).height == 4);
 }
 
+// The shell (editor_input_waveform.cpp) checks the loop crossfade's own grab handle — at
+// loopStart - crossfade — before it iterates the ordinary marker array, because a zero-length
+// fade puts that handle exactly on the loop-start marker's frame. The same coincidence recurs
+// whenever ANY marker shares that frame, most plausibly the START marker dragged up against the
+// fade edge: this module can't exercise the shell's check-order itself, but it can prove the
+// geometric ambiguity that makes the ordering load-bearing — the array's own first-match rule
+// would otherwise resolve the top strip to the START marker, not the fade handle.
+static void testStartMarkerSharesTheHandleStripWhenItSitsAtTheFadeEdge() {
+    const Rect a = wideArea();
+    const std::int64_t loopStart = 400, crossfade = 30;
+    const std::int64_t fadeEdge = loopStart - crossfade;  // where the crossfade handle sits
+    const std::int64_t markers[3] = {fadeEdge, loopStart, loopStart + 100};  // start dialled here
+    const int mx = frameToX(overlayOf(a), 1000, fadeEdge);
+    const int topY = a.y;  // inside the handle's top strip
+    // Without the shell's priority check, the array's own first-match rule already resolves the
+    // column to the start marker (index 0) at this x/y...
+    CHECK(markerAtPoint(overlayOf(a), 1000, markers, 3, mx, topY) == 0);
+    // ...and the fade handle's rect claims the exact same pixel — the ambiguity the shell
+    // resolves by asking the handle first, same as it does for the zero-fade/loop-start case.
+    CHECK(contains(markerHandleRect(overlayOf(a), 1000, fadeEdge), mx, topY));
+}
+
 // --- Per-lane envelope content -------------------------------------------------
 
 static void testAsymmetricStereoLanesCarryDifferentContent() {
@@ -435,6 +457,7 @@ int main() {
     testCoincidentMarkersStayIndependentlyGrabbable();
     testMarkerHandleClipsIntoTheArea();
     testMarkerHandleOnDegenerateAreas();
+    testStartMarkerSharesTheHandleStripWhenItSitsAtTheFadeEdge();
 
     testAsymmetricStereoLanesCarryDifferentContent();
     testLaneEnvelopeRejectsOutOfRangeLane();