Encode transfer-data attachments as raw IPC attachments instead of first
rewrapping them as IPC::File values.
This is preparatory refactoring for the upcoming Mach-port transport
introduction on macOS, where attachments should remain transport-native
rather than being normalized through file descriptors.
Replace IPC::File / AutoCloseFileDescriptor / MessageFileType in
the IPC message pipeline with a new IPC::Attachment class. This
wraps a file descriptor transferred alongside IPC messages, and
provides a clean extension point for platform-specific transport
mechanisms (e.g., Mach ports on macOS) that will be introduced later.