[davidackerman]

Sets the version to 0.1.0a2 and gets the changelog ready to publish it.

• Version. pyproject.toml, chunkmirage/__init__.py and uv.lock now say 0.1.0a2. tests/test_docs.py checks that they match.
• CHANGELOG. The top section was still headed "0.1.0a1 (unreleased)", but a1 went to PyPI on 2026-10-05.
  • The 0.1.0a1 section is restored to its text at the v0.1.0a1 tag.
  • Everything merged since a1 is under a new 0.1.0a2 section: the core/contrib split, worldarray, the corner-at-origin placement rule, a page's own Python in the browser engine, materialize, dataset://, coverage://, overlap and the rest.
  • The a2 section's intro says the plugin API only grew, with one exception: an ArrayInfo given no translation now moves by half a voxel.
• Release steps. docs/contributing.md now says a GitHub release runs publish.yml, replacing the old uv build && uv publish.

A local build makes a wheel that holds chunkmirage and chunkmirage_contrib and requires worldarray<1,>=0.1.

After merging, publish a GitHub release tagged v0.1.0a2 (marked as a pre-release) and publish.yml uploads it to PyPI.