SharedMemoryRegion gains a Win32 backend behind the unchanged public
API: Local\OakShm<key> names, OpenFileMapping for attach, VirtualQuery
for the size check, UnmapViewOfFile/CloseHandle for teardown. Semantic
differences from POSIX are documented: unlink_key is a no-op (the
kernel destroys the object with the last handle, so crashed owners
self-heal) and Create on a live name fails instead of replacing.
Windows CI builds the workspace again.