Enable Spot Newton MJWarp preset

[AntoineRichard]

Summary

• Enabled the Spot Newton MJWarp preset to use the Newton collision pipeline instead of MuJoCo contacts.
• Increased Spot MJWarp constraint/contact capacity and set the Newton shape margin used by other rough-terrain locomotion presets.

Test Plan

• ./isaaclab.sh -p -c "from isaaclab_tasks.core.velocity.config.spot.flat_env_cfg import PhysicsCfg; cfg = PhysicsCfg().newton_mjwarp; print(cfg.solver_cfg.njmax, cfg.solver_cfg.nconmax, cfg.solver_cfg.use_mujoco_contacts, cfg.collision_cfg.max_triangle_pairs, cfg.default_shape_cfg.margin)"
• SKIP=check-git-lfs-pointers ./isaaclab.sh -f
• ./isaaclab.sh -f (blocked locally because git-lfs is not installed)

[isaaclab-review-bot]

Review: Enable Spot Newton MJWarp preset

Verdict: ✅ Approve — clean, well-scoped configuration change

Summary

This PR enables the Newton collision pipeline for the Spot MJWarp preset by switching from MuJoCo-based contacts (use_mujoco_contacts=True) to Newton's own collision system, and tunes the associated solver/collision capacity parameters. The change is minimal (1 file, ~6 net additions) and well-targeted.

Analysis

Configuration Changes:

Parameter	Before	After	Assessment
njmax	45	130	~3× increase in constraint capacity — appropriate for Newton collision pipeline which generates more constraint rows than MuJoCo contacts for articulated robots
nconmax	30	40	Modest contact capacity increase
use_mujoco_contacts	(default: True)	False	Core change — enables Newton's own collision detection
collision_cfg	(not set)	NewtonCollisionPipelineCfg(max_triangle_pairs=2_500_000)	Configures triangle mesh collision budget for Newton pipeline
default_shape_cfg	(not set)	NewtonShapeCfg(margin=0.01)	Sets collision margin for shape expansion

Correctness:

• All new imports (NewtonCollisionPipelineCfg, NewtonShapeCfg) are used immediately — no dead code.
• Cross-referencing with Go2's config (njmax=65, nconmax=35, no Newton collision pipeline): Spot is a heavier, more complex quadruped with more contact surfaces, so the ~2× multiplier on njmax relative to Go2 is reasonable.
• margin=0.01 (1cm) is a standard value for locomotion tasks — small enough to avoid ghost contacts while providing adequate broadphase expansion.
• Setting use_mujoco_contacts=False while adding collision_cfg forms a coherent switch: the collision source changes from MuJoCo to Newton's native pipeline.

Performance Considerations:

• max_triangle_pairs=2_500_000 reserves GPU memory for collision pair buffers. For flat-terrain scenarios this is generous but safe — it won't degrade performance, just pre-allocates memory (~tens of MB).
• The increased njmax (130) means larger constraint Jacobian matrices per environment. This is well within typical GPU capacity for batched simulation.
• nconmax=40 is conservative — Newton clamps rather than crashes if exceeded, so no stability risk.

Missing Rough-Terrain Config:

• The Spot config directory has only flat_env_cfg.py (unlike Go2 which has both flat + rough). This is not a concern for this PR, but worth noting that a rough-terrain variant with Newton would likely need higher max_triangle_pairs.

CI Status

Check	Status
pre-commit	✅ Pass
Build Wheel	✅ Pass
Check for Broken Links	✅ Pass
Check changelog fragments	❌ Fail
Installation Tests	⏳ In progress

The changelog check failure is a repo policy requirement (not a code defect). A changelog fragment may be needed before merge.

Recommendation

Approve — this is a straightforward, well-reasoned configuration change that correctly enables Newton's collision pipeline for Spot. The parameter values are consistent with the codebase patterns and appropriately scaled for the robot's complexity.

[kellyguo11]

would be good to cherry pick this to the release branch as well