why: Cut the next alpha covering the iOS Safari SPA-nav sidebar-logo
fix (pushState ordering so relative asset URLs resolve against the
new page, not the old one — a WebKit-only resolution-timing bug).

what:
- just bump-version 0.0.1a12: every workspace package's
  pyproject.toml version, every __init__.py / extension.py
  __version__, the workspace pyproject.toml, scripts/ci helpers
  and matching test, and uv.lock all advance to 0.0.1a12.
- CHANGES gains a dated "## gp-sphinx 0.0.1a12 (2026-04-27)"
  release header above the existing 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.