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.

ConceptGraph

In-memory concept graph for query term expansion (Tier 3).

TierResult

Result from a single tier in the cascade.