[tx] Implement mHC

[pcmoritz]

The SkyRL tx JaxBackend is designed to be simple/hackable and make research easy. One technique that would be interesting to implement in this spirit is https://arxiv.org/pdf/2512.24880, and the fact that there is a common code base for inference and training should make it particularly suited for this task.

[tanmaysachan]

Implemented mHC in torch recently - taking up this issue
Thanks!