Add SDF-preserving gear asset culling script

[karanchahal-nv]

Summary

• Add scripts/forge/cull_gear_assets.py for copying an input gear USD asset tree and producing a reduced/cull variant.
• Uses Open3D quadric decimation plus duplicate/degenerate/unreferenced mesh cleanup per UsdGeom.Mesh.
• Preserves authored USD physics collision APIs and reports non-SDF collision approximations in a JSON manifest.

Motivation

GearAssembly profiling showed that reducing the prebaked SDF mesh triangle count materially lowers PhysX triangleTriangleCollision cost while preserving the SDF task constraint. This script makes that asset-prebake path repeatable from original/input gear assets.

Validation

• /home/horde/IsaacLab/isaaclab.sh -p -m py_compile scripts/forge/cull_gear_assets.py
• /home/horde/IsaacLab/isaaclab.sh -p scripts/forge/cull_gear_assets.py --help
• Smoke run against the existing reduced gear asset tree with --keep-factor 1.0 wrote /tmp/openclaw_test_culled_gears/manifest_reduction.json; manifest reported 32 meshes and non_sdf_collision_approximations: [].

Example

./isaaclab.sh -p scripts/forge/cull_gear_assets.py \
  --source /path/to/original_or_reduced_gear_assets \
  --variant reduced_gears_x20 \
  --keep-factor 0.5 \
  --min-faces 96 \
  --overwrite