docs(research): define model capability extension

[AVIDS2]

Scope

Adds the versioned boundary for a future model-capability moderation study:

• a plain-language explanation of what MemorixBench-Transfer currently tests;
• an explicit claim boundary: the current paper does not infer model-capability effects;
• a separate proposed design for calibrated model-agent configurations, task strata, and memory-condition interactions.

This is documentation only. It adds no experimental result or product-effect claim.

Relationship

This PR targets the research branch behind #136 so that the roadmap lands with the research artifact rather than duplicating that draft against main.