Extract category library routing

docs/prompt-architecture-improvement-plan.md

@@ -95,10 +95,14 @@ Keep here:
 
 Move or isolate later:
 
-- category-library loading and inheritance helpers into `category_library.py`.
+- pair assembly and camera mutation helpers that still live in
+  `prompt_builder.py`.
 
 Already isolated:
 
+- JSON category loading, subcategory normalization, named scene/expression/
+  composition pool loading, cast compatibility filtering, exact subcategory
+  lookup, and inheritance-based pool merging live in `category_library.py`.
 - hardcore configured-cast role graph generation lives in
   `hardcore_role_graphs.py`; `prompt_builder.py` selects item/axis metadata and
   then asks that module for the source role graph.