Expand evaluation harness for multi-document and metadata cases

[voidrot]

Summary

Expand the evaluation harness so compiler changes for multi-document output and metadata enrichment are measured before they become the default path.

Why this work is needed

The repo already has an evaluation harness, but it does not yet capture the new compiler behaviors planned under this epic.

Scope

• Add evaluation cases for multi-document outputs.
• Add evaluation cases for topic and metadata enrichment.
• Keep the harness useful for comparing compiler changes.
• Preserve compatibility with existing evaluation workflows.

Out of scope

• Hybrid retrieval evaluation.
• Frontend UX testing.

Acceptance criteria

• The evaluation harness covers representative multi-document cases.
• Metadata-enrichment behavior is represented in the dataset or checks.
• Compiler changes in this stream can be regression-tested.

Dependencies

• Depends on Epic: Compiler multi-document publishing and metadata enrichment #62.
• Depends on Refactor publish to emit multiple live documents #76.
• Depends on Add topic enrichment and metadata merge rules for publication #77.