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.
