Trim and validate scene caveats

docs/troubleshooting.md

@@ -183,6 +183,7 @@ Symptoms include:
 
 - `dnd scenes chunker`
 - `malformed structured output`
+- `boundary_caveats`
 - `start_unit_id`
 - `end_unit_id`
 - `gap`
@@ -199,6 +200,8 @@ Fix:
 - Inspect retained diagnostics for the run error and resolved pipeline.
 - If the error names malformed structured output, retry with a model that
   follows structured response schemas reliably.
+- If the error names `boundary_caveats`, check for blank or whitespace-only
+  caveat text in the scene response.
 - Scene boundaries must use exact source-unit IDs, cover the full source
   document, be contiguous, and not overlap.