feat(pylock): allow select() to prefer source distributions

[r266-tech]

Summary

Add the maintainer-proposed prefer_sdist_predicate argument to Pylock.select() so installers can apply per-package source-build policy.

• Select an available sdist before compatible wheels when the predicate returns True.
• Preserve compatible-wheel selection, sdist fallback, and the existing error when no usable source exists.
• Document the 26.3 API addition and cover the full wheel/sdist selection matrix.

Closes #1333.

Verification

• python -m pytest -q (62118 passed, 1 skipped, 417 deselected)
• python -m pytest tests/test_pylock_select.py -q (25 passed)
• ruff check src/packaging/pylock.py tests/test_pylock_select.py
• ruff format --check src/packaging/pylock.py tests/test_pylock_select.py
• uvx nox -s docs (392 doctests passed; HTML and LaTeX builds clean)
• Targeted strict mypy check for both changed files

[sbidoul]

For ease of verification, this should not change existing tests.

[r266-tech]

Thanks — updated in ad866d0.

• Restored test_sdist_fallback and test_missing_sdist_fallback unchanged.
• Moved the new behavior into a separate test_prefer_sdist_predicate parameterized test.
• Avoided calling the predicate when no sdist exists, and added the incompatible-wheel case to make the issue's source-preference contract explicit.

Verified with pytest tests/test_pylock_select.py -q (24 passed), Ruff check/format, and git diff --check.