[3mei]

Related issue

None.

What changed and why

Prepares the 0.3.0 release. One concern per commit.

• Which part of the version a release raises. The release steps in CONTRIBUTING.md say that, before 1.0, a release holding a breaking change (a commit whose title carries !) raises the minor version, and any other release the patch version. Since 0.2.0, sixteen commits carry !, so this release is 0.3.0. The steps also say that written notes replace a published release's generated ones: a summary, the highlights, and upgrade guidance that says what each breaking change asks of users.
• The version is 0.3.0. pyproject.toml and the project entry in uv.lock say 0.3.0. The README installs usersim>=0.3.0 and says Python 3.10 is supported through the 0.3 releases, and the extension package template suggests pinning usersim>=0.3,<0.4, as docs/extensions.md already does.

How it was verified

On every commit's own tree: make check-all, the pre-commit hooks on the commit's files, and uv run pytest --collect-only -q (4884 tests collected).

At the head, on its exact tree in a fresh environment:

$ uv lock --check
Resolved 188 packages in 3ms
$ make check-all
All checks passed!
417 files already formatted
$ make test
4884 passed in 84.28s (0:01:24)
$ make smoke
All checks passed.
$ make check-license-headers
   ❌ Need update: 0
   ✅ Up to date: 419
$ make check-doc-links
✓ every relative link, and every link into this repository's main branch, resolves
$ make check-notebooks-clean
OK 4 notebook(s) carry no outputs
$ make check-dist
✓ installed package runs from a clean environment
$ make check-extensions
7 passed in 5.36s
7 passed in 2.47s
$ python scripts/check_release_tag.py v0.3.0 dist --branch main
✓ v0.3.0 releases 0.3.0 from a commit on main
prerelease=false
$ usersim --version
usersim 0.3.0