Validate the external backend catalogs

docs/policy/architecture.md

@@ -29,6 +29,8 @@ The implemented internal components consist of:
   constructs the default execution target;
 - `internal/filecatalog`, which discovers YAML files and provides source-path
   helpers for filesystem and `fs.FS` consumers;
+- `internal/catalog`, which validates immutable external maintained-catalog
+  assets before they are eligible for engine assembly;
 - `internal/jsonvalue`, which validates and defensively copies JSON-compatible
   extra-parameter trees;
 - `internal/promptdef`, which loads and validates prompt definitions from