Make spline EG drags audible live: held and new notes follow a dragged knot

Contours ride their own seqlock block into a double-buffered engine copy, and the swap step is smoothed. Each drag move publishes contours without a model write; release still commits and reloads.

src/core/instrument/engine/live_contours.cpp

@@ -0,0 +1,16 @@
+// live_contours.cpp — the VelocityCurve -> fixed-capacity copy the contour block carries.
+
+#include "core/instrument/engine/live_contours.h"
+
+namespace reasampler::instrument::engine {
+
+ContourPoints contourPoints(const VelocityCurve& curve) {
+    ContourPoints out{};
+    const std::vector<VelocityPoint>& pts = curve.points();
+    const std::size_t n = pts.size() < kMaxCurvePoints ? pts.size() : kMaxCurvePoints;
+    for (std::size_t i = 0; i < n; ++i) out.points[i] = pts[i];
+    out.count = static_cast<std::uint32_t>(n);
+    return out;
+}
+
+}  // namespace reasampler::instrument::engine