fix(deps): require XXE-safe lxml

[rylinjames]

Fixes #314

What changed

• require lxml>=6.1.0 in the safety extra so URDF parsing cannot resolve to a vulnerable release
• update the generated lxml lock entry from 6.0.4 to 6.1.2
• add metadata and lockfile regression tests for the security floor

Security evidence

Tether passes user-selected URDF files into yourdfpy.URDF.load from the safety guard and embodiment adapter. With the former locked lxml 6.0.4, an isolated XML probe resolved a local-file external entity. With locked lxml 6.1.2, local_file_resolved=False.

Verification

• pytest -q tests/test_pyproject_metadata.py: 4 passed
• PYTHONPATH=src pytest -q tests/test_guard.py tests/test_adapt.py: 42 passed
• ruff check and ruff format --check: passed
• built wheel metadata: Requires-Dist: lxml>=6.1.0; extra == safety
• full local suite: 3544 passed, 84 skipped; 23 unrelated environment failures from incompatible locally installed Transformers/Hugging Face and partial LeRobot packages

Lockfile note

The repository lock root metadata is already stale relative to pyproject.toml. A full refresh is independently blocked by current optional-dependency resolution conflicts (ARM64 NumPy/LeRobot, unavailable scservo-sdk registry resolution, and NVIDIA TensorRT placeholder metadata on macOS). This PR keeps #314 narrow and updates only the generated lxml package stanza.