Polish the Python SDK first-run surface

[rmcdaniel]

Summary

• replace the oversized root README with a stable, runnable Python entry point while retaining the complete guide under docs/
• make the developer portal use stable package and Server channels
• remove obsolete prerelease resolution, duplicate docs evidence, path-classification, and CLI-only comparison automation
• retain the Python version matrix, Server integration, replay/codec corpus, package integrity, public-boundary checks, and one desktop/mobile portal pass

Verification

• 1300 passed, 2 skipped, 15 deselected
• Ruff and mypy pass
• strict MkDocs build and docstring cross-reference checks pass
• Cloudflare analytics check passes
• Chromium layout and interaction check passes at desktop and mobile viewports
• wheel and sdist pass Twine and installed-package smoke checks
• exact release metadata matches the committed source
• central action policy and public-boundary checks pass