feat: graphs now properly sized + port static graphs to c++

Plugins/ZShell/Internal/linegeometry.hpp

@@ -0,0 +1,64 @@
+#pragma once
+
+#include <qpoint.h>
+#include <qvector.h>
+
+#include <cmath>
+#include <cstdint>
+
+namespace ZShell::internal {
+
+// Shared curve construction for SparklineContent (QPainter) and LineGraphItem
+// (QCanvasPainter). Input and output coordinates are logical (item-local);
+// device-pixel quantization happens inside so a stroke lands on whole pixel
+// columns and its edges are not interpolated across several columns. Keeping the
+// math in one place means both graphs align and smooth their lines identically,
+// whatever backend they paint through.
+
+inline qint64 toDev(qreal logical, qreal dpr) {
+	return std::llround(logical * dpr);
+}
+
+inline qreal fromDev(qint64 device, qreal dpr) {
+	return qreal(device) / dpr;
+}
+
+// Snap a line center so a whole-pixel-wide stroke sits on pixel boundaries when
+// the device width is even and on pixel centers when it is odd.
+qreal snapLine(qreal logical, qint64 widthDev, qreal dpr);
+
+// Catmull-Rom control points for the segment points[i] -> points[i + 1], with the
+// control y clamped to the segment's own y span so a curve never overshoots past
+// its endpoints. When curveSmoothing is off (or there are too few points) the
+// tangents are flat.
+void segmentControls(
+	const QVector<QPointF>& points,
+	int i,
+	bool curveSmoothing,
+	QPointF& c1,
+	QPointF& c2);
+
+// y of the cubic bezier p1-c1-c2-p2 at targetX, found by bisection on t.
+qreal cubicY(
+	const QPointF& p1,
+	const QPointF& c1,
+	const QPointF& c2,
+	const QPointF& p2,
+	qreal targetX);
+
+// The curve point at targetX on the polyline, interpolating across the segment
+// that contains it. Assumes points ascend in x.
+QPointF pointAtX(
+	const QVector<QPointF>& points,
+	qreal targetX,
+	bool curveSmoothing);
+
+// A dense polyline through points, adding one vertex per device-pixel column
+// inside each segment so an edge never interpolates across several columns. The
+// result starts at points.first() and ends at points.last(). points must ascend in
+// x and hold at least two entries. Backends consume it as a moveTo/lineTo chain.
+QVector<QPointF> curveVertices(
+	const QVector<QPointF>& points,
+	qreal dpr,
+	bool curveSmoothing);
+} // namespace ZShell::internal