feat: project skeleton, core types and math with transform tests

include/plot/plot.hpp

@@ -0,0 +1,14 @@
+#pragma once
+
+/// Umbrella header: pulls in every public component of the plot library.
+/// Later layers (gl, data, render, plot, input) are added here as they land.
+
+#include <plot/core/color.hpp>
+#include <plot/core/rect.hpp>
+#include <plot/core/span.hpp>
+#include <plot/core/status.hpp>
+#include <plot/core/types.hpp>
+
+#include <plot/math/mat3.hpp>
+#include <plot/math/transform.hpp>
+#include <plot/math/vec2.hpp>