why: Cut the next alpha covering the SEO-packages branch (the two
new sphinx-gp-opengraph + sphinx-gp-sitemap packages, register-
aware autodoc-docutils discovery, and the post-audit bug fixes).

what:
- just bump-version 0.0.1a10: every workspace package's
  pyproject.toml version, every __init__.py _EXTENSION_VERSION
  / __version__, the workspace pyproject.toml, scripts/ci helpers
  and matching test, and uv.lock all advance to 0.0.1a10.
- CHANGES gains a dated "## gp-sphinx 0.0.1a10 (2026-04-25)"
  release header above the existing Breaking changes / What's new
  / Bug fixes content; the "## gp-sphinx 0.0.1 (unreleased)"
  placeholder + maintainer comment stays at the top of the file
  as the next-release insertion point.