Chunk streaming concurrency, entity slot fix, and map-data-driven spawns

- Allow 2 chunks to be mid-load concurrently instead of 1 (MAP_CHUNK_LOAD_CONCURRENCY).
- Fix entitySetChunk silently losing track of an entity when its target chunk's
  entity slots are full - it now stays detached (and retries later) instead of
  claiming a chunk that never actually registered it.
- DCF format bumped to v5: chunks can now declare entity spawns (global/NPC via
  the existing entityglobal registry, or one-shot item pickups) and map area
  triggers, resolved via a new callback-ID registry (mapareagloballist.h)
  mirroring the entity one. rpg.c's hardcoded TEST entity/item/area spawns are
  gone - chunk_0_0_0.json now carries that data instead. The player is still
  bootstrapped in code since it isn't map-authored content.

assetsraw/chunks/chunk_0_0_0.json

@@ -2179,5 +2179,27 @@
         0
       ]
     }
+  ],
+  "entities": [
+    {
+      "type": "global",
+      "globalId": 3,
+      "pos": [8, 8, 1]
+    },
+    {
+      "type": "item",
+      "itemId": 1,
+      "quantity": 1,
+      "pos": [12, 2, 0]
+    }
+  ],
+  "areas": [
+    {
+      "min": [11, 3, 0],
+      "max": [16, 9, 10],
+      "callbackId": 1,
+      "notify": 3,
+      "trigger": 6
+    }
   ]
 }