Fix arity and return value mismatches

`assert` returns every argument it is given, so `return assert(x, msg)`
was leaking the message as a second return value.

`get_tile` is documented as taking x and y. The async function class
declared `@operator call` with no parameters, which made every async
call look over supplied.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

exp_util/module/async.lua

@@ -94,7 +94,7 @@ Async.status = {}
 
 --- @class Async.AsyncFunction
 --- @field id string The id of this async function
---- @operator call: Async.AsyncReturn<any>
+--- @overload fun(...: any): Async.AsyncReturn<any>
 Async._function_prototype = {}
 
 Async._function_metatable = {