Implement integer source units and chunk payloads

internal/modules/extract/dnd/spells/testdata/seriatim_spell_session.json

@@ -5,21 +5,21 @@
   },
   "segments": [
     {
-      "id": "seg-001",
+      "id": 1,
       "start": 0,
       "end": 4,
       "speaker": "Alice",
       "text": "Aria raises her holy symbol and casts Cure Wounds."
     },
     {
-      "id": "seg-002",
+      "id": 2,
       "start": 4,
       "end": 8,
       "speaker": "DM",
       "text": "The bandit mage casts Shield as the blow lands."
     },
     {
-      "id": "seg-003",
+      "id": 3,
       "start": 8,
       "end": 12,
       "speaker": "Bob",