Promote the Unreleased changelog block to 9.1.0 and bump the package
version in pyproject.toml, uv.lock, and CITATION.cff.

The release covers everything on main since 9.0.0: the prebuilt fullmap
download with --force to rebuild (#83), named optional-extra errors with
their install commands (#82), annotation split_by sharing one splitting
primitive with explode_by (#79), the bundled [aria2] extra replacing a
system aria2c, and the GitHub issue/PR templates.

Also corrects documentation drift the release surfaced: llms.txt listed
the extras as `rt` and `qc` only (predating `aria2`, `agent`, and
`optimize`) and did not map the new tablassert.extras registry, the
docs/cli.md `--version` example was still `tablassert 8.2.0`, and the
_prebuilt_fullmap_urls docstring example named 9.0.0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wvqjv7dvCwexyWciPKMLk6