Add CLI argument parsing for test infrastructure: --test-mode enables
deterministic seed (42) and warn-level tracing to stderr, --port allows
OS-assigned ports (--port 0), --seed overrides RNG seed. Prints
LISTENING:{port} to stdout after bind for test harness discovery.
Extracts setup_proof_room() for reuse. Fixes #459.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>