fs: implement rename, symlinks, *_at methods, and path tracking for in_mem/tar_ro

[wdcui]

Summary

• in_mem.rs: Adds rename (with POSIX directory-into-descendant EINVAL check), symlink/link/read_link support, create_anonymous_file (memfd), path tracking on descriptors, all *_at methods (open_at, stat_at, unlink_at, readlink_at, rename_at, mkdir_at, fd_path), O_PATH flag handling, root UID 0 permission bypass (CAP_DAC_OVERRIDE), just_created flag to skip permission checks for file creators, root dir ownership changed to 1000:1000 for single-user sandbox model
• tar_ro.rs: Adds path tracking on descriptors, all *_at methods, O_PATH flag handling, rename/mkdir_at stubs returning ReadOnlyFileSystem
• errors.rs: New RenameError variants (IsADirectory, NotADirectory, NotEmpty, InvalidArgument, CrossDevice, SameFile, ClosedFd, ReadOnlyFileSystem), new ReadLinkError/SymlinkError/LinkError variants, error message corrections
• mod.rs: DirFdError enum for *_at helpers, memfd_display_path function; PTY trait methods deferred to layered.rs PR
• devices.rs: RenameError::NotSupported → RenameError::Io (variant removed in errors.rs)

Stack

• PR 7: wdcui/stacked/pr7-devices ← base
• PR 8: this PR
• PR 9: fs/layered.rs (next)

Testing

• Build: clean
• Clippy: clean
• Fmt: clean
• Tests: 208 passed, 15 failed (all pre-existing TUN/EPERM), 34 skipped
• Doc: clean