XY-886: [ELF benchmark adapter] Implement LightRAG Docker context-export adapter

[yvette-carlisle]

Summary

• add a Docker-contained LightRAG context-export smoke with explicit service/provider/storage configuration
• add a LightRAG mode to the real-world live adapter materializer that ingests generated source-mapped corpus texts and exports context/source mappings
• update the external adapter manifest while keeping non-executed suites typed as not_encoded and avoiding live pass claims

Validation

• cargo make lightrag-docker-context-smoke
• cargo make fmt
• cargo make lint-fix
• cargo make checks