feat: expand env-step benchmarks and fix fork-main PR CI

[TATP-233]

Summary

• add a detailed MuJoCo backend step benchmark that splits backend hot-path costs by component
• improve the existing env step benchmark plots and outputs for easier comparison across tasks and backends
• ensure PRs from fork repositories using a main head branch still trigger GitHub Actions instead of remaining silent

Linked Work

• Issue: none
• Milestone: none

Validation

• make check
• uv run pytest -m "not slow"
• Additional task-specific validation listed below

Commands actually run:

uv run python -c "import pathlib, yaml; yaml.safe_load(pathlib.Path('.github/workflows/ci.yml').read_text()); print('ci.yml ok')"
uv run pytest tests/scripts/test_check_docs.py -q

Impact

• Backend impact: none
• Platform impact: both
• Training effect expected: no

Artifacts

• W&B: none
• benchmark result: not attached
• video / screenshot: none
• ONNX / checkpoint: none

Checklist

• Added or updated tests where needed
• Updated docs if behavior or workflow changed
• Linked the driving issue
• Noted any follow-up work explicitly