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

include/plot/core/span.hpp

@@ -0,0 +1,11 @@
+#pragma once
+#include <cstddef>
+#include <span>
+
+namespace plot {
+
+/// `std::span` re-exported so public headers and callers share one spelling.
+template <class T, std::size_t Extent = std::dynamic_extent>
+using Span = std::span<T, Extent>;
+
+}  // namespace plot