Prepare v0.2.1 release

[nekrasovp]

Summary

• set project, package, and lock metadata to 0.2.1
• finalize the idle connection manager fix changelog entry as 0.2.1 dated 2026-08-26
• keep the release-preparation diff limited to metadata; no runtime or public API changes

Prerequisite verification

• push CI and Pages both passed on exact base commit a3717e2

Validation

• uv sync --group dev
• uv lock --check
• uv run ruff check .
• uv run mypy
• uv run pytest -q -s (58 passed)
• uv run mkdocs build --strict
• uv build
• uv run twine check dist/*
• Python 3.12 fresh-venv wheel install/import/version smoke
• full gate repeated on exact commit in a git clone --no-local checkout

Release boundary

This PR does not create or push the v0.2.1 tag, create a GitHub release, publish to PyPI, or merge itself.