Function verify_graph 

pub fn verify_graph(nodes: &RecordBatch, edges: &RecordBatch) -> GraphViolations

Verify graph coherence of merged CodeNode + CodeEdge RecordBatches.

Checks:

• No duplicate node_id values in the CodeNodes batch
• No dangling edge sources (every source_id exists in CodeNodes)
• No dangling target_id values — ext: refs are excluded (they are intentional unresolved external references)