Reintroduce multiple maps: per-map chunk directories + map_t name/mapSetMap

Move chunk assets from a single flat assetsraw/chunks (and assets/chunks)
into per-map assetsraw/maps/<map>/chunks (assets/maps/<map>/chunks), so
terrain mesh/model output no longer collides across maps sharing chunk
coordinates. map_t gains a name[MAP_NAME_MAX] field plus mapSetMap()/
mapIsLoaded() (checks name[0] == '\0') in place of the old bool loaded
flag; mapInit() now just resets state and mapSetMap() does the chunk
grid load. Updated the chunk asset tool and the map editor's dev
server/client to match.

editor/server/server.py

@@ -209,7 +209,10 @@ class Handler(BaseHTTPRequestHandler):
             self.send_json({"error": "invalid chunk coordinates"}, 400)
             return
 
-        json_path = PROJECT_ROOT / "assetsraw" / "chunks" / f"chunk_{x}_{y}_{z}.json"
+        json_path = (
+            PROJECT_ROOT / "assetsraw" / "maps" / "overworld" / "chunks"
+            / f"chunk_{x}_{y}_{z}.json"
+        )
         if not json_path.is_file():
             self.send_json({"error": "chunk json not found"}, 404)
             return