feat: add Python checker parity (BodyContact/PositionDelta/OrderedStep)

[keyneszeng]

Summary

Adds the Python counterparts of the 3 new JS checkers (companion PRs to AxisWebInfra), wires them into run_checker(), and ships 27 unit tests.

Changes

• New util/check_body_contact.py — AABB intersection + inflation
• New util/check_position_delta.py — per-axis delta vs baseline
• New util/check_ordered_step.py — walks a trajectory with a step cursor
• New tests/test_body_contact.py (7 cases), tests/test_position_delta.py (11 cases), tests/test_ordered_step.py (9 cases)
• Modified util/validate_offline_trajectory.py — imports the 3 modules and dispatches them in run_checker based on type

Cross-repo contract

JS and Python are documented to use the same task-config field names and the same semantic outcome for each checker. The Python side is the source of truth for offline trajectory validation; the JS side is the source of truth for in-browser teleoperation. Any change to the algorithm must update both.

Implementation notes

• BodyContact: AABB intersection (no live MuJoCo needed offline). Considers objects in state.object_positions and optional state.object_extents; default half-extent 2.5 cm.
• PositionDelta: requires an explicit initialPosition (or state["initial_position"]) as the baseline. Without a baseline it conservatively returns False (matching the JS side's first-call behavior where captureRuntimeInitial: true means delta = 0 until movement is observed).
• OrderedStep: walks a trajectory once with a cursor. The caller must pass the full trajectory under state["_trajectory"] (a list of state dicts); without it, the check returns False rather than guessing.

Verification

• 27/27 unit tests pass
• run_checker dispatch verified: existing 10 types still work + 3 new types correctly routed

Run with::

python -m pytest tests/ -v