v0.2.0: release: prepare 0.2.0

Fifteen issues (#28–#42), filed after reading the whole upstream docxtpl
tracker and asking which of its open reports this architecture could answer.

- `__version__` becomes `0.2.0`, and `HISTORY.md` gains the heading and date.
  A minor bump rather than a patch: twelve of the fifteen add public API.

The release notes lead with the three that turned out to be worth more than
the report that prompted them — the render cache, which came from measuring
that Jinja2 compilation is 86% of a render where the phases everyone assumes
are the problem are 5%; the run-style check, which had to move from where the
issue proposed it because a Text holds no template; and lists, where defining
numbering per list dissolved both constraints that looked unavoidable.

They also state the one behaviour change plainly rather than burying it:
`StyleError` for a run style the document cannot carry. Only styles a render
*introduces* are checked, so nothing that renders today starts failing — but a
template that was silently producing unstyled text now says so, and someone
upgrading should know that before they do.

Verified: 479 tests pass on 3.12, ruff and pyright are clean, mkdocs builds in
strict mode, and `make check` reports both distributions well formed.

Pushing this does not publish anything — that takes a `v0.2.0` tag.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>