Implement integer source units and chunk payloads

docs/integrations/dnd-spell-artifacts.md

@@ -44,8 +44,8 @@ Approved artifacts use the generic artifact envelope documented in
   "source_refs": [
     {
       "source_id": "session-alpha",
-      "start_unit_id": "seg-001",
-      "end_unit_id": "seg-001"
+      "start_unit_id": 1,
+      "end_unit_id": 1
     }
   ]
 }
@@ -78,7 +78,7 @@ Each source reference uses the generic source-reference shape:
 Validation requires:
 
 - at least one source reference;
-- non-empty source ID and unit IDs;
+- non-empty source ID and positive unit IDs;
 - source ID matching the source document ID;
 - start and end unit IDs existing in the source document;
 - start unit appearing before or at the same position as end unit.
@@ -98,8 +98,8 @@ The extractor asks the LLM for this top-level response shape:
       "source_refs": [
         {
           "source_id": "session-alpha",
-          "start_unit_id": "seg-001",
-          "end_unit_id": "seg-001"
+          "start_unit_id": 1,
+          "end_unit_id": 1
         }
       ]
     }