Epic: The resource model #444

Specs: 4-resource-management.md ·
4.1-resource-identity.md ·
4.2-mem-resource.md ·
4.3-resource-registration.md ·
4.4-root-path-triad.md
Plans: resource-management.md ·
resource-provider.md ·
drop-resource-id-from-constructor.md

Resources are the identity layer the graph reasons about: what a unit touches, how it is named, how two
references to the same thing are known to be the same, and how the catalog is owned and handed to the
sealed graph.