Consolidate RrfFusion with generic CbrFusion

[mdproctor]

Phase 3 (#83) introduces CbrFusion — a generic fusion utility parameterized over . The existing RrfFusion in rag-api is RetrievedChunk-specific. Migrate QueryExpandingCaseRetriever and ReactiveQueryExpandingCaseRetriever to use CbrFusion, then remove RrfFusion.

References:

• rag-api/src/main/java/io/casehub/neocortex/rag/RrfFusion.java
• rag-expansion/src/main/java/io/casehub/neocortex/rag/expansion/QueryExpandingCaseRetriever.java
• rag-expansion/src/main/java/io/casehub/neocortex/rag/expansion/ReactiveQueryExpandingCaseRetriever.java