Update D&D schemas to require integer unit_id values

docs/troubleshooting.md

@@ -263,7 +263,7 @@ Fix:
   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
+- Scene boundaries must resolve to valid source units, cover the full source
   document, be contiguous, and not overlap.
 
 ## Session ID