Why: A model-visible flag cannot prove that a person confirmed one proposal revision and payload hash.
Evidence: go test ./pkg/extension ./internal/app ./internal/server -run 'HumanDecision|SurfaceSnapshot|ResolveHuman|NaturalLanguage' passed locally.
Impact: SurfaceVersion 2.22.0-experimental adds HumanDecisionVerifier. Existing Host methods stay unchanged, and the base does not consume the proof.
Refs: W4b human confirmation design r1
Co-authored-by: Cursor <cursoragent@cursor.com>
