Make xdoctest pytest items safe for embedding

[Erotemic]

What changed

• Allows XDoctestItem to be embedded when the optional xdoctest_namespace fixture is not registered.
• Preserves real fixture dependency failures instead of swallowing every FixtureLookupError.
• Defers .rst and .txt collection only when the active pytest-doctestplus collector actually owns the path through pytest's standard doctest globs.
• Rebuilds the PR directly on current main, excluding stale checker-lifecycle and module-metadata implementations inherited by the old stacked branch.
• Keeps the embedding tests executable across the pytest 6 / Python 3.8 and current-pytest lanes.

Pytest compatibility corrections

• The embedded collector resolves path on modern pytest and falls back to legacy fspath instead of assuming either API or using a platform-sensitive relative filename.
• The text-file ownership unit test passes pytest's real legacy py.path type on pytest 6 and pathlib.Path on pytest 7+, matching the production hook boundary.
• Runtime annotations in the tests remain Python 3.8-safe.

Skip behavior

Current main already contains the accepted #213 behavior: DocTest.run() reports full skips with the most concrete metadata reason, and XDoctestItem.runtest() retains the empty-run fallback. This PR deliberately leaves that centralized behavior unchanged rather than adding a second skip path.

Validation

• The complete pytest embedding/coexistence group passes in the available environment; the real pytest-doctestplus case skips when that package is absent.

• An explicit legacy pytest ownership probe exercises the py.path.check(fnmatch=...) branch.

• The publisher runs the complete embedding group under python3.8 when available.

• The publisher requires mypy src/xdoctest, ty check src/xdoctest tests/, fatal flake8 selectors, and git diff --check before publication.

• Uses a real PathLike argument for legacy py.path.local.join, satisfying both pytest 6 runtime behavior and current ty stubs.