Add abstract MemoryBase base class with typed Read<T>/Write<T>, array
IO, string IO, and relative/absolute addressing. Uses MarshalCache<T>
to branch between blittable (MemoryMarshal) and marshal-required paths.
Add ExternalReader (out-of-process via ReadProcessMemory/WriteProcessMemory)
with SafeMemoryHandle lifecycle management.
11 new tests: ImageBase, Read/Write of int/byte/long/struct, byte array,
int array, struct array, invalid address, dispose, double-dispose.
All passing (total: 30).
