Address backend implementation review findings

docs/policy/architecture.md

@@ -27,6 +27,8 @@ 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/jsonvalue`, which validates and defensively copies JSON-compatible
+  extra-parameter trees;
 - `internal/promptdef`, which loads and validates prompt definitions from
   filesystem and `fs.FS` sources;
 - `internal/profile`, which loads, validates, and overlays execution profiles
@@ -76,9 +78,11 @@ downstream consumers, including Scriptorium
         narrow injected abstractions
 ```
 
-The backend registry depends on the domain model, has no mutation API after
-construction, and shares its OpenAI-compatible reserved request-field rule
-with the model client. The facade coordinates internal components and adapts
+The backend registry depends on the domain model and shared JSON-value
+validation, has no mutation API after construction, and consumes the
+OpenAI-compatible reserved request-field rule owned by the model client. The
+model client does not depend on registry configuration. The facade coordinates
+internal components and adapts
 the supported public extension interfaces to narrow internal abstractions.
 Internal components must not depend on consumers or on Scriptorium.