Fix Call.target_type() for variadic builtin.tuple

builtin.tuple is registered as variadic (arity -1), so make_builtin
gives it an empty param list and the generic type-variable matching
can't resolve the return type T. Add a special case that infers
TupleType directly from the argument types, matching what _infer_type
already does in the parser.

Without this, IfElse nodes wrapping tuple constructs were typed as
interface{} in Go, causing a compile error when the function's declared
return type is []interface{}.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: davidwzhao

meta/src/meta/target.py

@@ -345,6 +345,9 @@ def __post_init__(self):
     def target_type(self) -> "TargetType":
         if isinstance(self.func, (ParseNonterminal, PrintNonterminal)):
             return self.func.target_type()
+        # builtin.tuple is variadic; infer TupleType from the argument types directly.
+        if isinstance(self.func, Builtin) and self.func.name == "tuple":
+            return TupleType(tuple(a.target_type() for a in self.args))
         func_type = self.func.target_type()
         if isinstance(func_type, FunctionType):
             # Match parameter types against argument types to build type variable mapping