fix: close review findings on spline EGs — engine, codec, and popup/overlay UI grammar

Live pitch depth, Gate/Spline enable-rule agreement, inert kTrigLength, NaN wire guards, hard-flag-tail corruption no longer wipes the record, RT/cold spline tie-break, retired alt-click, marker-shadow fix, plus new test coverage.

src/shell/instrument/editor_input_curve.cpp

@@ -45,17 +45,8 @@ void ReaSamplerEditor::handleCurveMouseDown(const Rect& r, int x, int y) {
     const VelocityCurve::Box box = curveBoxFromRect(r);
     if (box.width <= 0 || box.height <= 1) return;
 
-    // Alt-click delete predates the converged grammar and is kept as a landed alternate; every
-    // other gesture routes through the shared resolver so both spline consumers stay one
-    // grammar.
-    if ((GetKeyState(VK_MENU) & 0x8000) != 0) {
-        const int alt = editedCurve().pointAtPixel(box, x, y);
-        if (alt >= 0 && editedCurve().deletePoint(static_cast<std::size_t>(alt))) {
-            hover_ = HoverTarget{};  // a stale kCurveNode index would light a shifted node
-            commitAndReload();
-        }
-        return;
-    }
+    // Alt-click delete is retired (the spec's right-click supersedes it — one grammar, no
+    // migration on either side): every gesture here routes through the shared resolver.
     const bool ctrl = (GetKeyState(VK_CONTROL) & 0x8000) != 0;
     const SplineEdit edit = resolveSplineEdit(
         editedCurve(), box, ctrl ? SplineGesture::kControlLeft : SplineGesture::kLeft, x, y);