Preserve inertial semantics when decoding USD

[LouRohanNV]

Summary

This PR preserves MuJoCo inertial semantics during USD decode:

• If UsdPhysicsMassAPI authors both mass and diagonal inertia on a body, mark the decoded body inertial as explicit.
• Decode UsdPhysicsMassAPI mass/density on visual UsdGeomGprim geoms as well as colliders.
• Treat visual-only MjcImageableAPI geoms with no authored mass or density as density=0, so they do not inherit MuJoCo default density and alter inferred body inertias.

Fixes #3399

Why

MuJoCo roundtrip validation showed that visual-only meshes could accidentally gain mass during USD import, and explicit inertials could be lost during compilation. Both affect sampled dynamics.

USD / non-USD impact

The change is contained to plugin/usd_decoder, which is only built when MUJOCO_WITH_USD=ON.

Validation

Roundtrip evidence:

• Franka hand/finger mass and inertia match after preserving explicit inertials.
• UMI gripper no longer gains mass from visual-only mesh geoms after defaulting massless MjcImageableAPI geoms to density=0.