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.h

@@ -0,0 +1,34 @@
+// curve_tessellate.h — the staged envelope's TRACE: envelope_overlay's node vertices joined by
+// the curve each stage's exponent defines. Split from that module on the axis the two already
+// have — envelope_overlay decides where a node LANDS, this strokes the span BETWEEN two of
+// them over phi, so a re-scaled time axis changes nothing here.
+
+#pragma once
+
+#include <vector>
+
+#include "core/instrument/ui/envelope_overlay.h"
+#include "core/ui/stroke_aa.h"  // StrokePoint — the stroker's own vertex type
+
+namespace reasampler::instrument::ui {
+
+// The exponent governing the segment that ENDS at `node`: attack, decay and release are the
+// three sloped stages. Every other node ends a plateau, whose straightness comes from its own
+// zero level span rather than from an exponent, so neutral is returned and no caller needs a
+// second rule to recognize one.
+double segmentCurve(const StageEnvelope& env, EnvNode endNode);
+
+// Replaces `out` with the polyline the shell strokes. Knot vertices are handles, not line
+// vertices, and are skipped; node vertices keep their exact INTEGER coordinates (clamped to
+// [xLo, xHi]) because those are the positions their draggable handles are drawn at. Only the
+// interior samples are sub-pixel, one per pixel column, which is what makes the density follow
+// the canvas width instead of a fixed count.
+//
+// A segment's level runs start + (end - start) * curveMap(phi, exponent) — the composition
+// envelopes.h's four evaluators use, so the trace cannot diverge from the sound. A neutral
+// exponent or a zero level span emits the two endpoints and nothing between: the straight
+// stroke, vertex for vertex.
+void buildEnvelopeTrace(const std::vector<EnvVertex>& poly, const StageEnvelope& env, int xLo,
+                        int xHi, std::vector<reasampler::ui::StrokePoint>& out);
+
+}  // namespace reasampler::instrument::ui