What to run as players move through a server zone.
A @:structInit class rather than a typedef, for the same reason as its
client counterpart: Haxe's Lua target treats a function-typed field of an
anonymous structure as a method and wraps the assigned closure in an
adapter emitted as function(_, ...) return function(...) ... end(...) end
— and calling a function literal without parentheses around it is a Lua
syntax error, so the script fails to load. The object-literal call site is
unchanged.