Add MarshalCache<T> static class that computes Size, SizeU, TypeRequiresMarshal, IsIntPtr, TypeCode, and RealType once per type in the static constructor. Handles bool (size=1), enums (underlying type), and MarshalAs-attributed fields (TypeRequiresMarshal). 12 new tests covering: blittable sizes, bool size, enum size, struct size, marshal-required flag, IsIntPtr, computed-once caching. All passing.