mujoco: Implement joint velocity commands

[azeey]

🎉 New feature

Toward #299

Summary

This PR implements joint velocity commands for the MuJoCo physics engine via native velocity actuators (mjBIAS_AFFINE).

To make velocity tracking configuration-independent and uniform, we dynamically compute the servo gain at each step as $k_v = J / \tau_s$, where $J$ is the un-factored joint inertia from d->qM[m->dof_Madr[dofIndex]] and $\tau_s = 0.05 \cdot \text{timestep}$ (5% of the timestep). This implements a proportional velocity controller using MuJoCo's native velocity actuators (mjBIAS_AFFINE).

Additionally, instead of using joint-level effort limits (actfrcrange), which are solved as soft constraints in MuJoCo and introduce compliance and phase lag, we set force limits directly on the actuator (forcerange) using the SDF effort limits. This enforces limits as a hard input saturation clip, which prevents solver compliance and keeps the control loop stable.

Backport Policy

• This is safe to backport to the following versions:
  • Jetty
  • Ionic
  • Harmonic
  • Fortress
• This should not be backported
• I am not sure
• Other (fill in yourself)

Test it

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the feature
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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Generated-by: Gemini 3.5 Flash

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