test(maps): maps_t2-t7 slot-resolution proofs for cell-tile

Proves that previously-unreachable atlas slots become reachable
via the cell-tile material path: slot 0 isolated (t3), slot 1 end
with correct rotations (t4), slot 4 straight (t5), slot 5
tee_open (t6), slot 8 cross_open (t7).

Upgrades write_test.map.json to schema_version 3 with material
and vertices fields on surface so cell_material_slot + set_vertex
resolve correctly in the tests.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

maps/write_test.map.json

@@ -1,10 +1,18 @@
 {
-  "schema_version": 2,
+  "schema_version": 3,
   "id": "write_test",
   "size": { "w": 4, "h": 4 },
   "atlases": ["demo_tilemap"],
   "layers": {
     "surface": {
+      "material": "demo_tilemap",
+      "vertices": [
+        0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0,
+        0, 0, 0, 0, 0
+      ],
       "tiles": [
         0, 0, 0, 0,
         0, 0, 0, 0,