Implementing an environment

Place a new environment under the appropriate backend with config.rs, env.rs, and mod.rs; keep model assets and backend helpers beside it.

1. Define a configuration with documented defaults and meaningful builder methods where fields are private.
2. Define exact State, Action, and Info types and an EnvSpace when the dimensions are static.
3. Make seeded resets reproducible, clear the step counter, and initialize every backend state used by state().
4. Validate action kind, dimension, and bounds before applying it.
5. Return an Experience containing the pre-step and post-step observations and a correct Terminal value.
6. Keep task termination separate from the configured time limit.
7. Test valid transitions, invalid actions, boundaries, seeded resets, rewards, and simultaneous terminal/truncation behavior.
8. Export the environment and config from mod.rs and add a specification page using the common environment template.

For MuJoCo, gate exports behind the mujoco feature, embed the default XML, derive dimensions from the loaded model, and document required body/site names for custom XML.