Trim and validate scene caveats

docs/internal/modules.md

@@ -88,7 +88,8 @@ last, exact source-unit IDs, sequential contiguous scenes, and no overlap. It
 assigns chunk IDs such as `scene-000001` and stores scene metadata including
 title, primary mode, participants, summary, boundary note, confidence, boundary
 unit IDs, and unit count. Boundary caveats become warnings with reason code
-`scene_boundary_caveat`.
+`scene_boundary_caveat`. Whitespace-only caveats are treated as malformed
+structured output rather than silently dropped.
 
 Malformed model output fails explicitly rather than falling back to another
 chunker. The chunker exposes prompt and response-schema provenance through