Γ-W1-T3: staged contour traces draw the curve their exponent defines

New pure curve_tessellate joins the overlay's node vertices through curveMap,
one sample per pixel column; the knot no longer floats off its own trace.

src/core/instrument/ui/curve_tessellate.cpp

@@ -0,0 +1,65 @@
+// curve_tessellate.cpp — see curve_tessellate.h. Pure geometry; no host types.
+
+#include "core/instrument/ui/curve_tessellate.h"
+
+#include <algorithm>
+#include <cstdlib>
+
+#include "core/util/curve_law.h"
+
+namespace reasampler::instrument::ui {
+
+using StrokePoint = reasampler::ui::StrokePoint;
+
+namespace {
+
+StrokePoint pt(double x, double y) {
+    return StrokePoint{static_cast<float>(x), static_cast<float>(y)};
+}
+
+// The samples strictly BETWEEN two nodes. The endpoints are the caller's, so a shared node is
+// emitted once and the polyline carries no zero-length joint.
+void appendInterior(int x0, int y0, int x1, int y1, double exponent,
+                    std::vector<StrokePoint>& out) {
+    const int span = std::abs(x1 - x0);
+    if (span < 2 || y1 == y0 || exponent == util::kCurveNeutral) return;
+    const double dx = static_cast<double>(x1 - x0);
+    const double dy = static_cast<double>(y1 - y0);
+    for (int i = 1; i < span; ++i) {
+        // phi is exact at every column, so x lands on the integer column and the last interior
+        // sample is one column short of the end node.
+        const double phi = static_cast<double>(i) / static_cast<double>(span);
+        out.push_back(pt(static_cast<double>(x0) + dx * phi,
+                         static_cast<double>(y0) + dy * util::curveMap(phi, exponent)));
+    }
+}
+
+}  // namespace
+
+double segmentCurve(const StageEnvelope& env, EnvNode endNode) {
+    switch (endNode) {
+        case EnvNode::AttackEnd:  return env.attackCurve;
+        case EnvNode::DecayEnd:   return env.decayCurve;
+        case EnvNode::ReleaseEnd: return env.releaseCurve;
+        default:                  return util::kCurveNeutral;
+    }
+}
+
+void buildEnvelopeTrace(const std::vector<EnvVertex>& poly, const StageEnvelope& env, int xLo,
+                        int xHi, std::vector<StrokePoint>& out) {
+    out.clear();
+    bool started = false;
+    int px = 0;
+    int py = 0;
+    for (const EnvVertex& v : poly) {
+        if (v.knot) continue;
+        const int x = std::max(xLo, std::min(xHi, v.x));
+        if (started) appendInterior(px, py, x, v.y, segmentCurve(env, v.node), out);
+        out.push_back(pt(x, v.y));
+        started = true;
+        px = x;
+        py = v.y;
+    }
+}
+
+}  // namespace reasampler::instrument::ui