Fix load_textures overwriting tile_size with nil for variable-size atlases

When an atlas declares tile_size_px = null (variable-size tiles), the
load_textures call was overwriting the map's tile_size with nil, causing
an arithmetic error in draw_layer on the first render frame.

Guard the tile_size update so it only applies when tile_size_px is
non-nil; maps using variable-size atlases keep their tile_size from
the stub default (32) set at load time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

init.lua

@@ -687,8 +687,10 @@ function M.load_textures(asset_aliases)
         }
     end
 
-    -- Update m.tile_size from first atlas (assume uniform; first atlas wins)
+    -- Update m.tile_size from first atlas only when the atlas declares a uniform
-    if m.atlases[1] then
+    -- tile size. Variable-size atlases (tile_size_px = null) keep the tile_size
+    -- that was set by the stub or tilemap JSON at load time (typically 32).
+    if m.atlases[1] and m.atlases[1].tile_size_px ~= nil then
         m.tile_size = m.atlases[1].tile_size_px
     end
 end