Add asserts for set_cell_gid write API

Covers happy-path round-trip, bounds rejection (positive and negative),
unknown-layer rejection, and on-demand layer allocation.

maps/write_test.map.json

@@ -0,0 +1,16 @@
+{
+  "schema_version": 2,
+  "id": "write_test",
+  "size": { "w": 4, "h": 4 },
+  "atlases": ["demo_tilemap"],
+  "layers": {
+    "surface": {
+      "tiles": [
+        0, 0, 0, 0,
+        0, 0, 0, 0,
+        0, 0, 0, 0,
+        0, 0, 0, 0
+      ]
+    }
+  }
+}