Module cascade 

Cascading retrieval pipeline (WG-131).

Implements a 4-tier retrieval cascade:

1. BM25 keyword index (CPU-local, no API calls)
2. HDC hyperdimensional encoding (CPU-local, no API calls)
3. ConceptGraph ontology expansion (CPU-local, no API calls)
4. API embedding fallback (external API call)

The cascade eliminates 50-70% of embedding API calls by satisfying queries from CPU-local tiers before falling back to the API.

Structs

CascadeConfig

Configuration for the cascading retrieval pipeline.

CascadeResult

Final result from the cascading retrieval pipeline.

CascadeRetriever

Cascading retrieval orchestrator.

TierResult

Result from a single tier in the cascade.