docs(readme): benchmark-race hero, animated demos, Markdown conversion

Rewrite the README as Markdown with a measured-data wow layout and wire
up the assets and gates that keep it honest:

- README.md replaces README.rst: centered badge row, animated benchmark
  race hero, feature sections with animated SVG demos (colors, logs,
  multibar, parallel execution, pv-style CLI), verified stability
  numbers, retained reference content
- benchmarks: measure plain progressbar2 and progressbar2[fast]
  separately in scenarios A and C, add speedups to requirements,
  refresh results/report/chart against current develop
- scripts/render_race.py: deterministic race SVG generated from
  results.json, byte-drift gated in tox docs-demos next to
  render_demos --check
- new registry demos readme/colors, readme/parallel and readme/cli with
  capture tests, README code blocks test-synced to the example sources
- render_demos: drop rows that MultiBar clears so removed bars no
  longer linger as stale duplicate frames
- fix(bar): NullBar.finish() now flips _finished, so a NullBar inside a
  MultiBar context cannot hang the exit wait (regression test included)
- tox: new readme env builds the sdist and runs twine check --strict
  (with readme_renderer[md]) in CI

Author: wolph

.github/workflows/main.yml

@@ -66,6 +66,8 @@ jobs:
           - python-version: '3.14'
             tox-env: docs-linkcheck
             experimental: true
+          - python-version: '3.14'
+            tox-env: readme
           - python-version: '3.14'
             tox-env: ruff
           - python-version: '3.14'

MANIFEST.in

@@ -3,6 +3,6 @@ recursive-exclude *.pyo
 include CHANGES.rst
 include CONTRIBUTING.rst
 include LICENSE
-include README.rst
+include README.md
 include examples.py
 include pytest.ini