docs: sync post-v0.2.1 published state

[CoderDeltaLAN]

Summary:

• Updates README now that v0.2.1 is published as GitHub Release and PyPI package.
• Updates CHANGELOG Unreleased with the post-release documentation sync.

Validation:

• Local checks passed: 118 tests, Ruff, text hygiene, Git whitespace.
• Pre-push guard passed.

Release note:

• Does not create or move tags.
• Does not publish a release.
• Does not publish to PyPI.