Plans to rebase onto mujoco 3.x? (+ mjx_diffrax license file)

[kaplanyan]

Thanks for releasing diffmjx — we reproduced the paper's results (CFD sign repair, Tsit5 gradient calibration, exact toy system-ID recovery) and are using the approach for contact-parameter identification from video.

Two questions:

1. 3.x rebase roadmap: is a rebase of the mujoco fork onto current mainline
   (3.11.x) planned? We ported the JAX-impl patches (CFD straight-through,
   scan_loop, softjax primitive collision smoothing, derivative/inverse utilities)
   onto the 3.11.0 tag here: https://github.com/kaplanyan/mujoco/tree/diffmjx-3.11 —
   happy to upstream it or serve as a starting point. Notes from the port: several
   3.3.1-era patches are already absorbed in mainline (rne flg_acc, safe_div
   call sites, the sensor rne-postconstraint set, io enableflags handling); 3.11
   moves impl-specific options behind opt._impl, and the CFD straight-through
   tree-map needs restricting to float leaves (3.11 Data carries bool fields).
   We validated sign-correct FD-vs-AD friction gradients and Adam parameter
   recovery on the ported stack. One observation: with mjx_diffrax on 3.11 we
   needed looser PID tolerances (rtol ~1e-2) than on 3.3.1 to avoid max_steps
   in contact-rich segments.

2. mjx_diffrax license: the repo currently has no license file (diffmjx and
   softjax are Apache-2.0). Could one be added so downstream use is unambiguous?