[Feature]: Per-env domain randomization of geom contact solver params

Hi team — first, thanks for Genesis, it's been great to work with for large-batch RL.

I'm using Genesis for sim-to-real on a contact-rich in-hand manipulation task (a dexterous hand reorienting a cube), and ran into one gap in the per-environment domain-randomization surface that I wanted to raise — and offer to help with.

What already works nicely (per-env DR)

Genesis exposes several per-environment randomization knobs, which is exactly what sim2real needs:

• State-level, always per-env (RigidSolverState, shape (_B, ...)): friction_ratio, mass_shift, i_pos_shift → per-env friction / mass / COM.
• Model-level, opt-in via flags: batch_dofs_info / batch_links_info / batch_joints_info → per-env kp/kv, armature, frictionloss, damping, link inertia, and joint sol_params.

These cover most of the usual DR ranges and work well.

What seems to be missing

Per-env geom contact compliance (sol_params = [timeconst, dampratio, dmin, dmax, width, mid, power], i.e. the MuJoCo solref/solimp analog) does not appear to be randomizable per environment:

• In RigidSolver.set_sol_params(...), the geom branch hardcodes batched = False (only joints_idx/eqs_idx can be batched), so passing a (n_envs, ...) tensor with envs_idx for geoms raises "Expecting at most 1D tensor".
• There is no batch_geoms_info option (only batch_dofs_info / batch_links_info / batch_joints_info).
• RigidSolverState has friction_ratio per geom/env, but no analogous per-env field for sol_params — geom sol_params lives only in the shared geoms_info.

So contact stiffness/damping can only be set globally (shared across the whole batch), whereas friction can already vary per env.

(Version: genesis==1.0.0.)

Why it matters

For contact-rich manipulation, randomizing contact compliance per env (alongside friction) is one of the more important levers for closing the sim2real gap — it's a standard part of DR recipes elsewhere (e.g. MuJoCo solref/solimp randomization). Per-env friction alone covers tangential grip but not normal-direction stiffness/restitution, so the policy can overfit to the simulator's exact contact response.

Possible directions

A couple of options that would mirror existing patterns:

1. Add a per-env sol_params_ratio (or full per-env sol_params) field in geoms_state, mirroring how friction_ratio works, and have the contact constraint kernel read the per-env value — then expose it via a set_geoms_sol_params / envs_idx-aware setter.
2. Or add a batch_geoms_info option (parallel to batch_dofs_info) that makes geoms_info.sol_params per-env.

Is per-env geom sol_params something you'd consider supporting, or is there an existing path I've missed? Happy to help test, and I'd be glad to contribute a PR (option 1 looks like the smaller change since friction_ratio is a good template) if that'd be welcome.

Thanks!