chore(spawners): migrate spawn data to $type discriminator

- Rename "type" -> "$type" (first key, required for STJ polymorphism) in
  all 109 spawn JSON files (5,612 records).
- Convert "homeRange" + "location" -> "spawnBounds" object, dropping homeRange.
  Runtime keeps reading homeRange so un-migrated/external files still load.
- Add BoundsEquivalenceTests (hr=0/1/3/7) validating that the script's toBounds
  formula produces the same Rectangle3D as the runtime homeRange path.
- Add MigratedDataLoadTests loading a real migrated file as List<SpawnerDto>
  and round-tripping each DTO through ToSpawner().
- Add tools/spawner-json-migrate/migrate.mjs (offline Node script).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Distribution/Data/Spawns/shared/ilshenar/Spectre.json

@@ -1,23 +1,58 @@
-[
+[
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "70e1e800-e6dc-4f5c-a610-269930e6c327",
     "name": "Spawner (310)",
-    "location": [1981, 1059, -28],
+    "location": [
+      1981,
+      1059,
+      -28
+    ],
     "map": "Ilshenar",
     "count": 13,
     "minDelay": "00:05:00",
     "maxDelay": "00:10:00",
     "team": 0,
-    "homeRange": 40,
+    "spawnBounds": {
+      "x1": 1941,
+      "y1": 1019,
+      "z1": -128,
+      "x2": 2022,
+      "y2": 1100,
+      "z2": 128
+    },
     "walkingRange": 40,
     "entries": [
-      { "name": "Skeleton", "maxCount": 3, "probability": 100 },
-      { "name": "Shade", "maxCount": 2, "probability": 100 },
-      { "name": "Spectre", "maxCount": 2, "probability": 100 },
-      { "name": "Wraith", "maxCount": 2, "probability": 100 },
-      { "name": "Ghoul", "maxCount": 2, "probability": 100 },
-      { "name": "Zombie", "maxCount": 2, "probability": 100 }
+      {
+        "name": "Skeleton",
+        "maxCount": 3,
+        "probability": 100
+      },
+      {
+        "name": "Shade",
+        "maxCount": 2,
+        "probability": 100
+      },
+      {
+        "name": "Spectre",
+        "maxCount": 2,
+        "probability": 100
+      },
+      {
+        "name": "Wraith",
+        "maxCount": 2,
+        "probability": 100
+      },
+      {
+        "name": "Ghoul",
+        "maxCount": 2,
+        "probability": 100
+      },
+      {
+        "name": "Zombie",
+        "maxCount": 2,
+        "probability": 100
+      }
     ]
   }
 ]