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>

.emmyrc.json

@@ -22,13 +22,8 @@
         "globals": [ "__DebugAdapter", "__Profiler" ],
         "disable": [
             "assign-type-mismatch",
-            "missing-parameter",
             "param-type-mismatch",
             "preferred-local-alias",
-            "redefined-local",
-            "redundant-parameter",
-            "redundant-return-value",
-            "return-type-mismatch",
             "unnecessary-assert",
             "unnecessary-if",
             "unused"