[engine-failure] PROJ-504-evaluating-the-impact-of-variable-select @ tasks: RuntimeError

Automated report (spec 023 / FR-016) — an unexpected engine failure occurred while processing **PROJ-504-evaluating-the-impact-of-variable-select** at stage `tasks`.

```
RuntimeError: LLMReviewer[constraint_preservation]: review frontmatter has neither `verdict:` nor `concerns:`/`action_items:` — a contentless/truncated review MUST NOT count as acceptance (false-convergence guard). First 200 chars: '```yaml\n---\nreviewer_name: constraint_preservation\nreviewer_kind: llm\nstage: tasks\nartifact_path: projects/PROJ-504-evaluating-the-impact-of-variable-select/specs/001-evaluating-the-impact-of-variable'
```

The project remains schedulable; it will retry on its normal cadence and recover once the underlying defect is fixed. Closing this issue does not require any project-state change.

Start with the automated report for spec 023 / FR-016 and trace the LLMReviewer constraint_preservation handling at the tasks stage, using the referenced projects/PROJ-504-evaluating-the-impact-of-variable-select/specs/001-evaluating-the-impact-of-variable-... artifact path. Reproduce the malformed review-frontmatter failure and inspect how verdict, concerns, and action_items are validated. Done means contentless or truncated reviews cannot count as acceptance and the normal retry can recover.

Written by the indexing model from the issue text.