feat(maps): save_to_disk emits cells_material conditionally

cells_material is added to the per-layer JSON only when at least
one entry is non-zero, so v0.5.6-vintage maps with no cell-tile
content write byte-identical output through the 0.5.7
serializer. Maps with cell-tile content round-trip the array
through save and reload.

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

init.lua

@@ -1761,6 +1761,21 @@ local function serialize_map_v3(map)
             if layer.material  ~= nil then layer_out.material  = layer.material  end
             if layer.vertices  ~= nil then layer_out.vertices  = layer.vertices  end
             if layer.overrides ~= nil then layer_out.overrides = layer.overrides end
+            -- 0.5.7: emit cells_material only when at least one entry is
+            -- non-zero. This preserves byte-identical save output for
+            -- pre-0.5.7-content maps.
+            if layer.cells_material then
+                local any_set = false
+                for i = 1, #layer.cells_material do
+                    if layer.cells_material[i] and layer.cells_material[i] ~= 0 then
+                        any_set = true
+                        break
+                    end
+                end
+                if any_set then
+                    layer_out.cells_material = layer.cells_material
+                end
+            end
             out.layers[layer_name] = layer_out
         end
     end