Add the stdlib doctest adapter package

[Erotemic]

What changed

• Replaces the broad xdoctest.interop module with the specifically named xdoctest.stdlib_doctest adapter package.
• Groups all existing stdlib-doctest support under that package:
  • stdlib_doctest/__init__.py is the documented public namespace and executable quick start.
  • stdlib_doctest/_checker.py contains checker registration, resolution, and the stdlib-shaped OutputChecker.
  • stdlib_doctest/_optionflags.py contains stdlib optionflag registration and runtime-state translation.
  • stdlib_doctest/_convert.py contains doctest.Example / doctest.DocTest intake.
• Removes the old top-level implementation modules checker_facade.py, directive_facade.py, stdlib_compat.py, and interop.py rather than leaving forwarding wrappers.
• Keeps the currently published top-level xdoctest aliases for now, but sources them from the single package.
• Updates core call sites to name their current private dependencies on _checker and _optionflags explicitly. This is intentionally an organizational move, not yet a redesign of which dependencies belong in core.
• Renames the dedicated tests and checked-in Sphinx page to match stdlib_doctest.

Public package

import xdoctest.stdlib_doctest as stdlib_doctest

The package re-exports the existing stdlib-shaped checker, optionflag, runtime-state conversion, and object-intake APIs. Its __init__.py contains documentation, the executable quick start, re-exports, and __all__; implementation remains in private modules.

Validation

• 122 checker, directive, runner, intake, and warning tests passed; 2 environment-dependent tests skipped.
• All 19 doctests in xdoctest.stdlib_doctest passed, including the package quick start.
• Python 3.8 grammar and compilation checks passed.
• mypy src/xdoctest and ty check src/xdoctest tests/ passed before packaging.
• Fatal flake8 selectors and git diff --check passed before packaging.
• The checked-in Sphinx API page rendered offline with no new package or duplicate-object diagnostics before packaging.