Feature Request: Newton/MuJoCo and VBD Physics Backend Support

Summary

Add Newton/MuJoCo and VBD as first-class validated physics backends in simready-foundation.

Motivation

Newton is the designated next-generation physics backend in Isaac Sim and Isaac Lab, and it integrates MuJoCo Warp as its primary solver and supports VBD for deformables.
Developers transitioning from PhysX to Newton solvers are seeing issues since the assets validated against PhysX profiles can behave incorrectly when loaded into a Newton solver runtime.

For instance, there seem to be two structural properties of Newton/MuJoCo that are incompatible with the existing rules:

• Actuator schema — Newton joints require mjc:gain / mjc:bias attributes (the MuJoCo force equation). The UsdPhysics.DriveAPI stiffness/damping attributes that PhysX consumes are silently ignored by Newton.
• Articulation structure — MuJoCo expects a nested kinematic tree (child rigid bodies parented under parent rigid bodies). The flat articulation structure that PhysX uses is not valid for reduced-coordinate MuJoCo solvers.

Without backend-specific profiles in this repo, there is no automated way to validate that an asset is actually ready for Newton simulation before it enters a pipeline.

Requested Work

• New MJC requirement rules covering the two structural differences above (actuator schema, articulation nesting), plus any MuJoCo-specific collider schema rules.
• New feature variants — that swap in the MJC rules in place of their PhysX.
• New profiles — that wire in the MJC feature variants. VBD is a solver selection within Newton (not a separate schema dialect).

No files or tests are named in the issue. Start by locating the existing PhysX requirement rules, feature variants, and profiles, then compare how they are wired together. Done means Newton/MuJoCo actuator and articulation requirements, relevant collider rules, and profiles using the new variants are validated; VBD remains a Newton solver selection.