Add Arena Gear Assembly scene

[TuurStuyck]

Summary

Add Arena Gear Assembly scene

Detailed description

• Ports the Gear Assembly scene into Arena with the existing Droid embodiment.
• Adds source-parity gear assets, reset/reward/termination wiring, and Newton backend defaults.
• Adds a regression check for scene composition and Newton MJWarp solver configuration.

gear_assembly_openpi_hdri_grid_table_view.mp4