Add D&D NPC artifact contract and identity codec

internal/modules/dnd/codec/npcs/testdata/dnd_npcs.v1.json

@@ -0,0 +1,12 @@
+{
+  "npcs": [
+    {
+      "id": "npc:sha256:99a16589618a04f535a7d21fdcc71a0b1c05d22f752cd492065b1086d97bc3d7",
+      "name": "Mira Thorn",
+      "aliases": ["The Greencloak"],
+      "description": "A guarded ranger who watches the northern road.",
+      "relationships": [{"target": "Captain Vale", "relationship": "reports to"}],
+      "source_refs": [{"source_id": "session-alpha", "start_unit_id": 1, "end_unit_id": 2}]
+    }
+  ]
+}