ADR-0012: Rich artifacts hang off the markdown spine

Status: accepted (carried; the Book machinery in apps/book implements it)

Book pages are not limited to prose: Mermaid diagrams, tables, and standalone HTML explainers (served from apps/book/public/explainers/) are first-class. Every rich artifact has a searchable markdown page as its canonical, navigable unit — diagrams render inline; standalone HTML is linked from its paired page. Courses are authored in docs/domain-knowledge/course-* and copied to the site at build time.