why: sphinx-vite-builder consolidation has landed on main, the
follow-up CI ordering fix is in, and live deploy verification across
the consumer corpus (10 repos × *.git-pull.com) confirms the
unstyled-wheel regression is resolved. Promote the workspace to the
0.0.1a16 final release line.

what:
- bump 0.0.1a16.dev4 → 0.0.1a16 across all 16 workspace package
  pyproject.toml + __init__.py __version__ entries + uv.lock + the
  workspace root pyproject.toml + the smoke-test version assertions
- CHANGES: promote the unreleased section's entries under a new
  ## gp-sphinx 0.0.1a16 (2026-05-03) heading; reseed an empty
  ## gp-sphinx 0.0.1 (unreleased) above for the next dev cycle
- check-versions exits 0 with all packages aligned

note: the v0.0.1a16 tag is intentionally NOT pushed by this commit —
deferred so the tag-driven release.yml pipeline can fire on a
deliberate manual trigger after final review of the published wheels.