Refactor-Object-Mutation-Immutable

The model receives the prompt (and optional system message). The run uses scorer rubric_json_metrics with the JSON configuration below. Pass/fail and partial credit are determined entirely by that scorer against the model output; no human grading.

Return JSON only with keys refactor, rationale, tests. Refactor a Redux-style reducer that directly mutates the state object (e.g. state.count = state.count + 1) to use immutable spread updates ({...state, count: state.count + 1}) for every case. The returned state shape must be identical.

{
  "metrics": {
    "visible": {
      "checks": [
        {
          "contains": [
            "spread"
          ]
        },
        {
          "contains": [
            "immutable"
          ]
        },
        {
          "contains": [
            "reducer"
          ]
        }
      ]
    },
    "hidden": {
      "checks": [
        {
          "contains": [
            "no direct mutation"
          ]
        },
        {
          "contains": [
            "all cases covered"
          ]
        },
        {
          "contains": [
            "state shape preserved"
          ]
        }
      ]
    },
    "intent": {
      "checks": [
        {
          "contains": [
            "predictability"
          ]
        },
        {
          "contains": [
            "state isolation"
          ]
        },
        {
          "contains": [
            "pure function"
          ]
        }
      ]
    }
  }
}