extra property
A kind this package cannot know about without depending on whoever
defines it — sampleRegistry's own extra parameter, threaded through
rather than duplicated. A host that plays a real shipped level headless
(flutter3d_sim_mcp's own crypt) passes WidgetSurfaceKind() here the
same way flutter3d_demo_dungeon's main.dart does for the real app,
since a level naming a word this game's own vocabulary does not know
refuses to load rather than silently dropping the entity.
Implementation
final List<EntityKind> extra;