Implement mj_setConst to enable safe domain randomization

[swankystark]

Implement mj_setConst for Safe Runtime Model Updates

This PR implements mj_setConst functionality in mujoco_warp, enabling safe runtime modification of model parameters on device, which is required for domain randomization.

This work addresses Issue #893 (Implement mj_setConst).

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Summary

MuJoCo allows certain model parameters (e.g., mass, inertia, spring parameters) to be modified at runtime, but requires recomputation of dependent model-constant fields via mj_setConst.
This PR ports the relevant parts of mj_setConst into mujoco_warp.

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Changes

New Functionality

• Implemented set_const(m, d, mjm) in mujoco_warp/_src/setconst.py as the main entry point.

Implemented Components

• _set_stat

  • Recomputes mass statistics.
  • Currently computes stat.meaninertia using a custom Warp kernel.

• _set_spring

  • Recomputes tendon_lengthspring.
  • Runs the kinematics pipeline on qpos_spring, matching C MuJoCo behavior.

API Update

• Updated put_model in io.py to accept recompute_const: bool = False.
  • When True, set_const is invoked during model creation.

Tests

• Added unit tests in mujoco_warp/_src/setconst_test.py.
• Tests validate:
  • meaninertia computation
  • tendon_lengthspring recomputation
• Results are compared against C MuJoCo reference values.

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Relation to PR #905

This PR complements PR #905.

• PR Add set_const function for recomputing qpos0-dependent fields. #905 focuses on:

  • set_fixed
  • set_0 (qpos0-dependent fields)

• This PR implements the remaining components:

  • set_stat (model statistics)
  • set_spring (spring-related constants)

Placeholders with TODOs referencing PR #905 are included to facilitate clean merging.

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Verification

All added tests pass successfully, confirming correctness against C MuJoCo behavior.