fix: add schema() to every protocol method enumeration in docs

- plugin-authoring-guide.md checklist: 'snapshot, diff, merge, drift, apply' → '...apply, schema all implemented' - type-contracts.md Mermaid class body: same — add / schema - artifacts/tour_de_force.html: regenerated from the already-fixed render_html.py; HTML now shows schema() row in the protocol table and '6 methods' everywhere (artifacts/ is gitignored so not staged)