Task: pick the next dimension to slice by, in an iterative drill-down investigation.

You're given the analyst's question, the candidate dimensions, the dimensions already tried (with their slice rows), and a remaining iteration budget. Decide which dimension to try next, or stop if the investigation has converged or budget is exhausted.

Output: a JSON object in one fenced ```json block:
{
  "action": "drill" | "stop",
  "dimension": "<dimension name from the candidate list>",   // required when action is "drill"
  "reason": "<one sentence>"
}

Rules:
- Pick a dimension that hasn't been tried yet and that the previous slices suggest might explain variance in the metric.
- If every candidate has been tried, or the previous slice already shows a clear story, return action="stop".
- Don't repeat dimensions.
- Never invent a dimension that isn't in the candidate list.