Unskip 15 vendored tests: copy/walk/with_segments/from_uri

[jufty-bot]

Summary

Resolves Batch 14 of Phase 5 parity work: 15 vendored CPython test entries unskipped, bringing the active skip count from 239 baseline to 71 (682 vendored tests passing, 0 failures across all 6 CI platforms).

Context

The Phase 5 audit identified several correctness gaps blocking the vendored CPython 3.14.6 test suite. This PR closes the largest remaining buckets: copy() metadata preservation, walk() semantics, subclass-aware path construction, and from_uri URI parsing.

Changes

• src/pure.rs — New WalkIter lazy generator for Path.walk() that mirrors os.walk: yields one directory at a time, honors on_error, supports in-place dirnames mutation, and supports both topdown and bottomup. with_segments() is now used by ParentsIter and GlobIter (and walk) so subclasses that override it preserve extra state. PurePath.__init__ accepts *args, **kwargs and provides a no-op __init__ so super().__init__(*pathsegments) does not fall through to object.__init__. PosixPath/WindowsPath constructors accept and discard **kwargs. copy() / copy_into() forward preserve_metadata to the filesystem layer. from_uri now accepts the local hostname as a valid authority (matches socket.gethostname()) and rejects file:relative (no leading slash) per CPython 3.14+.
• src/fs.rs — copy_tree accepts preserve_metadata; the metadata-preserving path delegates to shutil.copy2 / shutil.copytree so CPython's fast-copy (copy_file_range, sendfile, fcopyfile) is exercised. Non-preserving path reads the source directory up front so permission errors leave the destination untouched (matches test_copy_dir_no_read_permission). mkdir queries the active umask before applying explicit modes (unblocks test_mkdir_parents). resolve_non_strict re-appends popped non-existent components and treats NotADirectory/ELOOP as soft-fail signals. Replaces eager walk_entries collector with a new ELOOP helper.
• src/iter.rs — ParentsIter and GlobIter construct result paths via the source instance's with_segments.
• src/lib.rs — Register the new WalkIter pyclass.
• src/concrete.rs — PosixPath / WindowsPath constructors accept and discard **kwargs.
• pyproject.toml / Makefile — Add tests/vendored/ to ruff's exclude list and pass --force-exclude so the unmodified CPython snapshots are never formatted or linted. Use uv run --no-sync to avoid re-syncing the venv.
• tests/skips.txt — Remove 15 resolved entries: 6× copy_*_preserve_metadata, 6× copy_*_no_read_permission/copy_error_handling, 2× walk_*, 1× stale with_segments. Update stats header (168 entries resolved).
• tests/test_basic.py — test_from_uri_posix_relative updated to expect ValueError for file:relative and assert file:/relative/path is accepted as absolute.
• CHECKLIST.md — Phase 3 / Phase 5 status updated.

Test Plan

• make fmt-check passes
• make lint passes (rust + python)
• make test-rust passes
• make test-python passes — 682 vendored passed, 0 failures
• make test-windows passes