Prepare 0.2.0b1 beta release

[iancharest]

Summary

• Bump package version to 0.2.0b1.
• Promote package classifier/status from pre-alpha to beta.
• Move current unreleased duplex work into the 0.2.0b1 changelog section.
• Record the ABI decision: keep per-CPython-version wheels for the public beta.
• Add a release checklist covering tag builds, TestPyPI, PyPI, and GitHub release artifacts.
• Document that Windows support is deferred until Windows test hardware is available.

Validation

• PyPI name check: https://pypi.org/pypi/tachyaudio/json returned 404, so the name appears available.
• PYTHONPATH=src python3 -m unittest discover -s tests
• python3 -m compileall -q src tests examples
• PYTHONPATH=src python3 - <<'PY' ... import tachyaudio ... PY returned 0.2.0b1.
• /private/tmp/tachyaudio-beta-venv/bin/python -m build --sdist --wheel
• /private/tmp/tachyaudio-beta-venv/bin/python -m twine check dist/*

Closes #8.