Feature: Retrieval Trace View for RAG Debugging

### Is your feature request related to a problem? Please describe.

When a workflow returns an incorrect or incomplete answer, it can be difficult to determine where the failure occurred.

For knowledge-base applications, users often need visibility into:

* Which chunks were retrieved
* Which chunks were selected after reranking
* Which sources were ultimately provided to the model

Without this information, debugging retrieval quality becomes largely trial-and-error.

### Describe the solution you'd like

Add an optional "Retrieval Trace" view for workflow executions.

For each query, display:

* Retrieved chunks
* Retrieval scores
* Reranked results
* Final context sent to the LLM
* Source attribution

This could be available in execution logs or a dedicated debugging panel.

### Describe alternatives you've considered

External observability tools can provide partial visibility, but they often lack workflow-specific retrieval information and require additional setup.

### Additional context

This feature would make it significantly easier to debug chunking strategies, retrieval quality, reranking performance, and knowledge-base coverage issues.