Plan traceable model-learning interactions

[Doppp]

Summary

• add a detailed implementation plan for model-visualisation course integration
• sequence exact replay, attention/backprop drill-down, Atlas topology and structural comparison, KV-cache lenses, and a gated prose-focus capability
• register the work as Milestone 12 in the canonical implementation plan
• preserve schema v1, the iframe security boundary, and evidence/disclosure semantics

Verification

• pnpm format:check
• documentation whitespace and link-resolution checks

Implementation is intentionally not included in this PR.