Summary

Replace hard-coded dist=1 sentinel values in MJX collision functions with actual signed distance values. This preserves the gradient chain through JAX even when geoms are not colliding, enabling differentiable distance computation.

Changes

All changes are in mjx/mujoco/mjx/_src/collision_convex.py (17 insertions, 15 deletions):

• _sphere_convex: Remove jp.where(has_separating_axis, 1.0, ...) — the expression d - sphere.size[0] is already correct for both cases
• _capsule_convex: Use actual face/edge distances instead of -1 sentinels
• _create_contact_manifold: Use jp.abs(penetration) for non-contact points instead of jp.ones_like
• _box_box_impl / _sat_gaussmap: Use jp.abs(dist) for inactive edge contact slots and face-separating cases
• plane_convex / hfield_*: Use jp.abs(support).max() / jp.max(dist) for non-unique contacts

Testing

• All 37 existing collision_driver_test.py tests pass
• Verified that jax.grad produces finite, non-zero gradients through contact.dist for non-colliding sphere-box pair

Fixes #3131