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

[wdcui]

Summary

Layer 1 of the core litebox crate changes. Refactors fd/mod.rs to support process-level operations (fork, dup, fd passing) needed by the upcoming process registry.

Part of the stacked PR series — base: PR #743 (foundation types)

Changes

fd/mod.rs (~396 insertions, ~153 deletions)

• on_dup() / on_close() hooks: Default-empty trait methods on FdEnabledSubsystemEntry for ref-counting subsystems (e.g., pipes, PTYs)
• close_and_remove: Replaces close_and_duplicate_if_shared with simpler always-remove semantics (Released variant replaces Duplicated)
• duplicate_raw_fd: Fork-time entry duplication by raw OwnedFd
• WeakEntryHandle + EntryHandle::downgrade(): Weak references to descriptor entries
• PassedFd / duplicate_for_passing / insert_passed_fd: SCM_RIGHTS-style fd passing between processes
• clone_for_fork on RawDescriptorStorage: Deep-clone descriptor table for fork
• entry_handles: Iterator over all alive entries of a subsystem
• EntryHandle: Restructured to named fields with Clone support, adds object_id() and identity_addr()

fd/tests.rs

• New tests for DescriptorObjectId preservation across duplicate and clone_for_fork
• Object ID assertions added to existing tests

net/mod.rs (minimal caller updates)

• close_and_duplicate_if_shared → close_and_remove
• CloseResult::Duplicated → CloseResult::Released
• Removed queued_for_closure field and attempt_to_close_queued() (no longer needed)

PR Stack

1. ✅ PR Syscall rewriter: interface changes, no_std support, and bug fixes #738 — Syscall rewriter interface (wdcui/stacked/pr1c-rewriter-interface)
2. ✅ PR Runtime ELF patching, trampoline format changes, and rtld_audit removal #739 — Runtime ELF patching + trampoline format (wdcui/stacked/pr1b-trampoline-format)
3. ✅ PR Cross-platform packager: OCI symlink resolution, rewrite-include, rtld_audit removal #741 — Cross-platform packager (wdcui/stacked/pr2-packager-crossplatform)
4. ✅ PR Foundation types: platform traits, event extensions, sync changes, AnyMap clone #743 — Foundation types (wdcui/stacked/pr3-foundation-types)
5. → This PR — File descriptor layer (wdcui/stacked/pr4-fd-layer)
6. 🔜 Layer 2: fs traits, mm ranges, net simplification, pipes