friction between cable and geometry (eg capstan)

### The feature, motivation and pitch

Hi MuJoCo team,

I’d like to request support for capstan friction in spatial tendons, which is important for cable-driven robotic hands. A simple model (T_high = T_low · exp(mu · theta)) would already be very useful.

Right now it’s hard to implement this in user code because spatial tendon wrapping does not expose wrap geometry (e.g., theta or entry/exit tangent points), and the tendon model effectively exposes only a single scalar tendon force—so users end up splitting tendons and manually approximating theta.

Could you either (1) add an optional built-in capstan friction model for cylinder/sphere wrapping, or (2) expose wrap geometry outputs (theta + entry/exit points) via API/sensors to enable robust plugins?

Thanks for considering,

### Alternatives

_No response_

### Additional context

_No response_