File descriptor layer: DescriptorObjectId, dup/close hooks, fork support, WeakEntryHandle

Refactor fd/mod.rs to support process-level operations (fork, dup, fd passing):

- Add DescriptorObjectId: monotonic u64 for open-file-description identity
- Add on_dup()/on_close() hooks on FdEnabledSubsystemEntry for ref-counting
- Replace close_and_duplicate_if_shared with close_and_remove (always-remove)
- Add duplicate_raw_fd for fork-time entry duplication
- Add WeakEntryHandle and EntryHandle::downgrade() for weak references
- Add PassedFd / duplicate_for_passing / insert_passed_fd for SCM_RIGHTS
- Add clone_for_fork on RawDescriptorStorage
- Add entry_handles iterator for all alive entries of a subsystem
- Restructure EntryHandle to named fields with Clone support
- Update net/mod.rs callers for close_and_remove / Released semantics

Author: wdcui