instrument: spline EGs — hard points on the one shared spline, a drawn contour per envelope beside its staged state, payload v13

src/core/instrument/ui/spline_edit.h

@@ -0,0 +1,42 @@
+// spline_edit.h — THE point-editing grammar, and the one place it is written down. Both spline
+// consumers route their mouse-down through it — the velocity-curve popup and the spline EG
+// overlay — so the two cannot drift into two grammars. Mirror of envelope_edit: decision logic
+// only, no host types, no drawing.
+
+#pragma once
+
+#include "core/instrument/engine/velocity_curve.h"
+#include "core/instrument/ui/editor_geometry.h"  // Rect / OverlayArea
+
+namespace reasampler::instrument::ui {
+
+using engine::VelocityCurve;
+
+// The gesture, in the pure module's own vocabulary (the shell maps its modifier state onto it).
+enum class SplineGesture { kLeft, kRight, kControlLeft };
+
+// What the gesture resolves to. Left-click adds a point in empty space and grabs an existing
+// one; right-click deletes; control-click toggles hard/smooth. Points are smooth by default.
+enum class SplineEditKind { kNone, kGrab, kAdd, kDelete, kToggleHard };
+
+struct SplineEdit {
+    SplineEditKind kind = SplineEditKind::kNone;
+    int index = -1;  // the point the action targets; -1 for kAdd (it has none yet) and kNone
+};
+
+// Resolves a click at (x, y) over `box` into an edit. The endpoint and point-count rules are
+// NOT re-stated here — kDelete on an endpoint and kAdd at the ceiling are refused by
+// VelocityCurve::deletePoint / addPoint, which the caller applies, so there is exactly one home
+// for each. A click outside the mapping box resolves to kNone unless it lands on a node's pick
+// radius: the drawn inset ring must not ADD (the new point would clamp onto an endpoint's x and
+// stack an undeletable duplicate) but must still be able to grab.
+SplineEdit resolveSplineEdit(const VelocityCurve& curve, const VelocityCurve::Box& box,
+                             SplineGesture gesture, int x, int y);
+
+// The contour's mapping box inside the waveform overlay: the FULL area, so the drawn contour
+// spans the whole sample width 1:1 with its time axis. No inset — unlike the popup's box, which
+// insets to keep endpoint handles clear of the sheet border, this one must stay 1:1 with the
+// waveform beneath it. Takes the overlay (not a lane) — see waveform_view.h's overlay contract.
+VelocityCurve::Box splineOverlayBox(const OverlayArea& area);
+
+}  // namespace reasampler::instrument::ui