Update D&D schemas to require integer unit_id values

internal/modules/chunk/dnd/scenes/assets/prompts/instructions.md

@@ -22,7 +22,8 @@ dnd/scenes boundary policy:
 - return sequential scenes with no gaps;
 - do not overlap scenes;
 - preserve source-unit order;
-- use exact source-unit IDs from the transcript;
+- use 1-based integer source-unit numbers from the transcript, where 1 is the
+  first provided source unit;
 - each scene must have start_unit_id and end_unit_id;
 - do not include final chunk IDs or chunk indexes.
 

internal/modules/chunk/dnd/scenes/assets/schemas/dnd_scenes.v1.json

@@ -26,12 +26,12 @@
         ],
         "properties": {
           "start_unit_id": {
-            "type": "string",
-            "minLength": 1
+            "type": "integer",
+            "minimum": 1
           },
           "end_unit_id": {
-            "type": "string",
-            "minLength": 1
+            "type": "integer",
+            "minimum": 1
           },
           "short_title": {
             "type": "string",