XY-882: [ELF research P2] Run D1/D2 adapter feasibility gates for RAG and graph memory systems

[yvette-carlisle]

Summary

• Record XY-882 D1/D2 feasibility verdicts for RAGFlow, LightRAG, GraphRAG, Graphiti/Zep, Letta, LangGraph, nanograph, llm-wiki, gbrain, and graphify.
• Keep all researched projects in research_gate evidence class; no live adapter pass evidence is claimed.
• Link implementation follow-ups only for adapter candidates: XY-885, XY-886, XY-887, XY-888, and XY-889.

Validation

• jq empty docs/research/2026-06-10-xy-882-rag-graph-adapter-feasibility.json apps/elf-eval/fixtures/real_world_external_adapters/memory_projects_manifest.json
• cargo nextest run -p elf-eval --test real_world_job_benchmark
• cargo make fmt
• cargo make lint-fix
• cargo make checks