Port the classic Brax/Gym locomotion tasks (walker2d, hopper, halfcheetah, ant, humanoid) to Playground physics

Hello !

Playground has become the natural successor to `brax/envs`: the physics is faster, the tooling is better, and the Brax README explicitly redirects users here:

> "Instead of `brax/envs`, users should use MuJoCo Playground, all of which train well with `brax/training`."

But, as far as I am aware, that's not quite true yet. Playground has the DMC versions (WalkerWalk, HopperHop, ...), which are close in spirit to Brax's gym-style locomotion envs but not the same task. They differ on reset, reward, and termination:

* DMC WalkerWalk resets pitch in [−π, π] and joints across their full range; Brax walker2d is init_q ± 5e-3.
* DMC reward is a shaped stand·walk product; Brax is `forward·x_velocity + healthy − ctrl_cost·‖a‖²`.
* DMC doesn't terminate on falling; Brax terminates when `torso_z ∉ [0.8, 2.0]` or `|angle| > 1.0`.

I ran into this on a safety-RL project and ended up writing a local wrapper stack to get Brax-walker2d semantics on top of Playground's WalkerWalk. Not hard, but it really should live upstream so we all share one implementation. This would also give conversations like #265 (reproducing PPO baselines on humanoid walk) a single tested reference point.

### What I'd propose

A subpackage (working name `brax_reborn`, `comebrax`, or whatever you prefer) with the five classic envs: Walker2d, Hopper, HalfCheetah, Ant, Humanoid.

Each port uses Brax's original MJCF (the MuJoCo XML robot description), Brax's `init_q + reset_noise_scale` reset, Brax reward with default weights, and Brax healthy-range termination. Physics is MJX, with both JAX and Warp backends via Playground's existing `impl` config.

### Faithfulness tests

For each env, roll a random uniform policy for N episodes in Brax (positional backend) and in the port with matching seeds. Assert that episode return, episode length, and termination rate distributions match within a loose tolerance. MJX and Brax's positional pipeline won't agree to the digit, but a gross mismatch catches the porting bugs worth catching. Also assert the obs and action dims match Brax.

### If this is of interest

I'm happy to do Walker2d end-to-end as a first PR (I have a prototype locally from the safety-RL project), and follow up with the other four if the shape looks right. Before I start, a few questions:

1. Is this something you'd want upstream, or leaning toward keeping it external?
2. Preferred name and location for the subpackage?
3. Reuse Brax's MJCFs verbatim, or re-author fresh ones? Verbatim is easier; re-authoring sidesteps any licensing question.

Cheers,

Yann

Start by comparing the existing Playground DMC WalkerWalk entry point with Brax's walker2d environment and the Playground `impl` backend configuration. Define the subpackage shape and implement one faithful Walker2d port first, then add the proposed random-policy comparisons for episode return, length, termination, and observation/action dimensions.

Written by the indexing model from the issue text.