Add GetAbsolute/GetRelative with nint-based arithmetic (bitness-agnostic).
Add 6 tests: absolute resolution, relative computation, base-round-trip,
isRelative flag on Read/Write/ReadBytes.
Fix MemoryBase.cs: restore for-loop body damaged by prior edit, clean up
StructureToByteArray overloads.
All passing (total: 44).
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).