[xuu33030]

Summary

Include tox.ini in the source distribution alongside the tests already shipped there. Without it, downstream users cannot run the documented tox environments from an unpacked sdist.

The change is a single explicit entry in MANIFEST.in; it does not change runtime behavior or the test configuration.

Closes #317.

Verification

On macOS with CPython 3.13.5:

• Built the unmodified sdist using python -m build --sdist --no-isolation and checked the archive: tox.ini was absent (the artifact check failed as expected).
• Rebuilt with this change and ran the same artifact check: exactly one tox.ini, byte-identical to the repository file. Comparing archive member lists showed only tox.ini added, with no members removed.
• Unpacked the fixed sdist and ran tox -e py313: 637 passed, 1 skipped.
• From the same unpacked sdist, ran tox -e doctest313: 80 passed.
• git diff --check passed.

The remaining Python/platform matrix and memory/coverage environments were not run locally. The artifact check was an external verification script, not a new test-suite dependency.

AI assistance: this patch and its verification were prepared with an AI coding assistant.