Enforce module family import boundaries

docs/internal/modules.md

@@ -189,6 +189,11 @@ Seriatim registrars own their production leaf registrations. The D&D registrar
 owns D&D leaf registrations, the spell default-validator chain, and D&D
 prompt/schema asset collection.
 
+Concrete implementation packages do not import generic implementation
+packages directly. A concrete family's `register` package is its composition
+point for specializing reusable generic implementations, while the generic
+registrar composes only generic children.
+
 Framework packages must not import production extensions. Tests may compose
 registries and catalogs directly with fakes.