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.

assets/maps/overworld/models/chunks/chunk_-1_0_0_0.json

@@ -0,0 +1,10 @@
+{
+  "mesh": "maps/overworld/meshes/chunks/chunk_-1_0_0_0.dmf",
+  "color": [
+    255,
+    255,
+    255,
+    255
+  ],
+  "texture": "tiles.png"
+}

assets/maps/overworld/models/chunks/chunk_0_0_0_0.json

@@ -0,0 +1,10 @@
+{
+  "mesh": "maps/overworld/meshes/chunks/chunk_0_0_0_0.dmf",
+  "color": [
+    255,
+    255,
+    255,
+    255
+  ],
+  "texture": "tiles.png"
+}

assets/maps/overworld/models/chunks/chunk_0_1_0_0.json

@@ -0,0 +1,10 @@
+{
+  "mesh": "maps/overworld/meshes/chunks/chunk_0_1_0_0.dmf",
+  "color": [
+    255,
+    255,
+    255,
+    255
+  ],
+  "texture": "tiles.png"
+}

assets/maps/overworld/models/chunks/chunk_1_0_0_0.json

@@ -0,0 +1,10 @@
+{
+  "mesh": "maps/overworld/meshes/chunks/chunk_1_0_0_0.dmf",
+  "color": [
+    255,
+    255,
+    255,
+    255
+  ],
+  "texture": "tiles.png"
+}

assets/maps/overworld/models/chunks/chunk_1_1_0_0.json

@@ -0,0 +1,10 @@
+{
+  "mesh": "maps/overworld/meshes/chunks/chunk_1_1_0_0.dmf",
+  "color": [
+    255,
+    255,
+    255,
+    255
+  ],
+  "texture": "tiles.png"
+}

assets/maps/overworld/models/chunks/chunk_2_0_0_0.json

@@ -0,0 +1,10 @@
+{
+  "mesh": "maps/overworld/meshes/chunks/chunk_2_0_0_0.dmf",
+  "color": [
+    255,
+    255,
+    255,
+    255
+  ],
+  "texture": "tiles.png"
+}