Centralize D&D chunk prompt material preparation

docs/internal/modules.md

@@ -58,6 +58,13 @@ resolution and an ephemeral NPC registry before lane-specific material; the
 NPC prompt appends its task and instructions. Stage contracts expose only
 Notarius structured-completion types, not Scriptorium public types.
 
+The shared `ChunkPromptMaterial` helper owns common transcript material
+preparation for the spell, NPC, and combat-turn extractors. It clones supplied
+source metadata, falls back to the materialized chunk when content is absent,
+checks that content remains chunk-identical, and fills only the common default
+fields. Extractors retain their request validation and wrap helper errors with
+their module context.
+
 Reference material may inform a module or prompt but must not become source
 evidence. The resolver and materializer behavior is described in
 [Pipeline Internals](pipeline.md#reference-materialization).