Render sites in the viewer (group 4+)

[siddhss5]

Problem

mj_viser only renders geoms (groups 0-5). MuJoCo sites are not rendered at all. Sites are commonly used for:

• End-effector frames (ee_site, tool_attachment_site)
• Sensor locations (ft_sensor_site)
• Target markers (mouth, grasp targets)

MuJoCo convention: sites default to group 4. The native MuJoCo viewer renders them as small colored shapes.

Proposed

Render sites as small spheres/frames in the viewer, respecting their group visibility toggle. Include group labels in the visibility panel:

• Group 0: default geoms
• Group 2: visual meshes (render only)
• Group 3: collision meshes (physics only)
• Group 4: sites (frames, markers)
• Group 5: tendons, sensors

Workaround

Add a small visual geom at the same position as the site with `contype="0" conaffinity="0" group="2"`.