fix: formalize backend play and snapshot capabilities

[TATP-233]

Summary

• formalize env-facing play/render/physics snapshot capabilities instead of letting training scripts touch directly
• route shared play helpers and through the env playback contract
• preserve backend-specific behavior: MuJoCo remains physics-snapshot/video oriented, Motrix remains native interactive-renderer oriented, without requiring feature parity
• add helper/script tests around the new contract and unsupported snapshot behavior

Validation

• uv run pytest tests/training/test_training_helpers.py tests/scripts/test_train_scripts.py -q

Fixes #222