x.executor: inline JobFn as fn () ! to avoid alias collision with x.async (#31191)

When a program imports both x.executor and x.async, the transform stage resolved executor.ExecutorJob.f (typed JobFn) to async.JobFn (fn (mut context.Context) !), because both modules declare a type alias named JobFn and the transform's type cache reused the async entry. Inlining the concrete signature removes the indirection and the collision.

vlib/x/executor/executor.v

@@ -18,7 +18,7 @@ mut:
 }
 
 struct ExecutorJob {
-	f            JobFn @[required]
+	f            fn () ! @[required]
 	result_ch    chan ExecutorJobResult
 	has_result   bool
 	report_error bool