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

include/plot/core/rect.hpp

@@ -0,0 +1,22 @@
+#pragma once
+#include <plot/core/types.hpp>
+
+namespace plot {
+
+/// Axis-aligned rectangle in pixels, origin at the top-left.
+struct Rect {
+  f32 x = 0.0f;
+  f32 y = 0.0f;
+  f32 w = 0.0f;
+  f32 h = 0.0f;
+
+  constexpr f32 right() const noexcept { return x + w; }
+  constexpr f32 bottom() const noexcept { return y + h; }
+
+  /// Half-open containment: the right/bottom edges are outside the rect.
+  constexpr bool contains(f32 px, f32 py) const noexcept {
+    return px >= x && px < right() && py >= y && py < bottom();
+  }
+};
+
+}  // namespace plot