test: use-case coverage + end-to-end smoke test
- tests/test_coven.py: capacity cap (5th rejected, configurable), duplicate
username, roster frame contents, slot/username freed on disconnect.
- tests/conftest.py: set app.ctx.max_users (fixes fixture vs new server code).
- hh/smoke.sh: one-command e2e — rust unit tests, SRP self-test, boot server,
rust handshake round-trip, cross-language python decrypt of a rust message.
- hh: drop unused Session.user_id (clean build).
pytest: 85 passed. smoke: PASS.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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