v0.2.18

Packaging and types, after an audit of the whole repository.

• py.typed ships, so a consumer's type checker reads the annotations that were already there. mypy is clean and runs in CI, which it did not before; it found 28 errors, nearly all from criteria being declared Mapping | Sequence[str] while __post_init__ normalizes it to a mapping.
• The base install is httpx alone. FastAPI and uvicorn moved to the server extra, where decide serve already told people to look. A client no longer installs a web server it never runs.
• Every README link is absolute, so the PyPI page renders: its diagram was a broken image and every link to docs/, the changelog and the licence was a 404, because PyPI resolves relative paths against pypi.org.
• Removed Capabilities.max_choice_options, which nothing set and nothing read, and an unused helper in decide.recording.