[engine-failure] PROJ-420-predicting-the-effect-of-alloying-on-the @ tasks: RuntimeError

Automated report (spec 023 / FR-016) — an unexpected engine failure occurred while processing **PROJ-420-predicting-the-effect-of-alloying-on-the** 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-420-predicting-the-effect-of-alloying-on-the/specs/001-predicting-the-effect-of-alloying'
```

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 constraint_preservation LLMReviewer handling at the tasks stage and the review artifact under projects/PROJ-420-predicting-the-effect-of-alloying-on-the/specs/001-predicting-the-effect-of-alloying... named in the report. Reproduce the retry path for spec 023 / FR-016 and verify that a truncated or contentless review without verdict, concerns, or action_items cannot count as acceptance.

Written by the indexing model from the issue text.