XY-841: [ELF research vNext] Refresh external memory capability map for real-world benchmark dimensions

[yvette-carlisle]

Summary

• Add a June 9 real-world benchmark-dimension map for reviewed external memory projects, including suite roles, fair adapter evidence, evidence class, confidence, and where ELF is not yet the reference.
• Update the external research inventory with benchmark-dimension roles and keep RAGFlow, LightRAG, and GraphRAG as D0 watch items with no strength claims.
• Add a machine-readable research run record and align the README latest external research refresh link/date.

Validation

• jq empty docs/research/2026-06-08-agent-memory-selection.json docs/research/2026-06-09-xy-841-external-memory-benchmark-dimensions.json
• Markdown table pipe-count check over README and updated research docs
• Semantic-drift rg checks against README, production adoption report, live benchmark runbook, and updated research docs
• cargo make fmt
• cargo make lint-fix
• cargo make checks