Adds runtime deps (playwright, mitmproxy, anthropic, genson) and dev
deps (pytest, pytest-asyncio); creates the tests/ scaffold with
fixture_site.py, conftest.py, and a smoke test. Documents the
mitmproxy aioquic/mitmproxy-rs stub workaround needed for
free-threaded CPython 3.14.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
