[family collaborator] have mentor view the workspace read-only with no accidental writes possible

Prove the full collaborator-by-link mental model end-to-end: a friend who pastes a URL while the primary is editing does not lose either edit, two edits on disjoint bullets land independently, two edits on the same bullet merge character-by-character via Y.Text, knockout/score disagreements resolve via last-write-wins on the structured scalar, kanban reorders under contention converge, a mentor joins read-only without writing, and a five-participant workspace still serializes correctly. P2 is the two-actor base; P3 stresses three-to-five actors on the same fields.

Proof set for Mentor opens read-only view — cannot accidentally toggle selection