Move the collision geoms to geom group 3

[T-K-233]

Summary

The URDF import leaves every collision geom in geom group 0. Group 0 is also the group of the scene's floor. A ray cast that looks for the terrain in group 0 therefore hits the robot too.

lite-motion-tracking's velocity task has such a ray cast. Its foot height scan casts 6 rays down from each sole and keeps the smallest height. When the swing foot passes the stance foot, a ray lands on the stance foot, and the scan reads the gap to its top face instead of the gap to the ground. The foot_clearance reward and the critic's foot_height observation use this reading.

urdf_to_mjcf now moves every collision geom to group 3 (COLLISION_GROUP), as MuJoCo Menagerie and mjlab's G1 do. The geoms keep colliding. Viewers and mjlab's ray sensors see groups 0 to 2 by default, so they no longer see the collision geoms. The visual meshes stay in group 1.

Variant	Collision geoms moved
lite	17
lite_pro	17
lite_dummy	12
lite_bimanual	11
lite_biped	26
lite_biped_debug	6

Impact

• Training: lite-motion-tracking's velocity foot scan sees only the ground. On a trained biped velocity policy (512 training envs), 0.3% of swing-phase readings were too low before: 1.8 cm median, 24 cm at worst, almost all on the other foot. With this change, 0.0% are wrong. lite-motion-tracking needs a lock update (uv lock --upgrade-package lite-description) to pick this up.
• Viewers: the MuJoCo viewer hides collision geoms by default. Turn on group 3 to show them.
• Physics: no change. Contacts do not depend on the geom group.

Test plan

• With the old code, a regeneration of all six MJCFs is byte-identical with MuJoCo 3.6.0, so the generator output is reproducible.
• With the new code, the group attribute of the 89 collision geoms is the only change. Every other element and attribute is identical.
• New test test_set_collision_group_moves_collision_geoms_only covers the post-process step.
• New test test_committed_mjcf_keeps_group_0_for_the_scene loads every committed MJCF and requires each robot collision geom in group 3, and the floor of a *_scene.xml in group 0. It failed on all 8 MJCFs before the regeneration.
• pytest: 1599 passed, 36 skipped.
• lite-motion-tracking, with this checkout on PYTHONPATH: all 26 tasks build, and the foot height scan reads 0.0% wrong.

🤖 Generated with Claude Code

https://claude.ai/code/session_01X43rjzmEQRBQHqbFhzEfXC