XY-897: [ELF benchmark vNext P3] Build competitor-strength evidence matrix contract

[yvette-carlisle]

Summary

• Add the June 11 competitor-strength evidence matrix report for all tracked memory/RAG/graph projects.
• Add a machine-readable JSON matrix companion with project, scenario, state, blocker, and follow-up measurement fields.
• Link the new report from the benchmarking guide index.

Validation

• python3 -m json.tool docs/research/2026-06-11-xy-897-competitor-strength-matrix.json
• JSON/Markdown/manifest consistency check: 17 projects, 14 scenarios, 21 adapter records
• cargo make fmt
• cargo make lint-fix
• cargo make checks