Add search explainability outputs

[aurexav]

Context
Debugging memory failures is hard without visibility into why a result ranked highly.

Goal
Expose compact explainability data for each result to support evaluation and tuning.

Scope

• Return component scores (retrieval score, rerank score, boosts).
• Include matched terms or fields when available.
• Provide a stable result handle for follow-up inspection.

Acceptance Criteria

• Developers can identify ranking causes with a single search response.

References

• https://github.com/tobi/qmd