Replace the auto-converted README.md (which carried pandoc title-ref
spans and a garbled Tests heading from a malformed rst underline) with
a clean conversion of README.rst, and remove the rst version.
pyproject.toml already declares README.md as the package readme.
Content is unchanged; bringing it up to date is left for a separate
change.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
(cherry picked from commit fa8adc30ca)
