chore(pyproject): register pytest.mark.live marker

Silences the PytestUnknownMarkWarning emitted by test_visual_mma.py and
test_visual_sim_gui_ux.py (3 instances). The @pytest.mark.live mark
already exists in the test files; pyproject.toml just didn't know
about it.

- pyproject.toml: added 'live: marks tests as live visualization tests
  (not in CI by default)' to [tool.pytest.ini_options].markers

pyproject.toml

@@ -44,6 +44,7 @@ markers = [
     "integration: marks tests as integration tests (requires live GUI)",
     "clean_install: clean install verification (opt-in via RUN_CLEAN_INSTALL_TEST=1)",
     "docker: docker build and run test (opt-in via RUN_DOCKER_TEST=1)",
+    "live: marks tests as live visualization tests (not in CI by default)",
 ]
 
 [tool.mypy]