Reject invalid replacement filenames in with_name

[00200200]

Path('data/report.txt').with_name('../other.txt') currently returns
data/../other.txt, although replacing a filename should reject directory
components. Empty names, '.', and Windows drive components are also accepted.

Validate the replacement name through the configured path module before
constructing the result. This preserves POSIX backslashes as literal filename
characters and rejects both Windows separator forms and drive-qualified names.
with_stem inherits the validation through its existing call to with_name.

Refs #214. This change only covers replacement-name validation; it does not
claim complete pathlib compatibility or close the compatibility checklist.

Validation on macOS, Python 3.11.5:

• New tests on unchanged main: 13 failed (missing ValueError), 3 passed.
• Same focused tests after the fix: 16 passed.
• Tests and module doctests: 151 passed, 8 skipped; one existing pytest fixture
  deprecation warning. Plugin autoload was disabled and filterwarnings cleared
  to run the functional suite without optional plugin dependencies.
• Ruff format check and git diff --check passed.
• Ruff check of the two changed Python files reports the same 21 diagnostics
  as unchanged main, with no new diagnostics. This is not a clean lint pass.

Focused command:
PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python3 -m pytest tests/test_path.py -k 'with_name or with_stem' -q -o addopts= -o filterwarnings= --tb=short

Full functional command:
PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python3 -m pytest tests/ path/ -q --doctest-modules -o addopts= -o filterwarnings= --tb=short

POSIX and Windows path parsing are covered via posixpath and ntpath. Native
Windows execution and other Python versions were not run.

AI disclosure: Implementation, tests, and this description were prepared with
OpenAI Codex assistance. The checks above were executed locally.

[Wkhanex]

Replacement names with directory/drive components are rejected under both POSIX and Windows path modules, including Windows drive-relative names. Valid sibling filenames still keep the original directory.

Checked 3db027d99702 with standalone Python 3.12.14 checks (13 passing assertions). I haven't run the full upstream suite.