Reuse origin metadata across configuration collections

docs/internal/sources.md

@@ -82,6 +82,9 @@ declaration; absent and null fields remain distinguishable from explicit
 strings, including empty strings. It closes content-file handles before
 returning and retains no source capability for public inspection.
 
+Prompt materialization shares a lazy metadata snapshot across root and collection
+resources during construction, avoiding a full metadata copy per collection.
+
 Role and cache-control normalization is applied to execution-owned message
 copies. ConfigKit v0.8.0 retains effective input message metadata and its
 resource origins; recording a sequence-wide normalization in the retained