Review coverage sweep over call shapes found a real gap: the
method_type_params stamp lived inside the extension-only branch, so an
explicit Pack<int>(5) could not split an ordinary generic/non-generic
overload pair — the type-arg filter saw two non-generic candidates and
declined, leaving declaration order to win. Hoist the stamp out of the
extension block; rides the same csharp@12 salt.
Also pins named arguments and out-var arguments as counted call shapes
(both already handled by csharpCallArgCount — argument nodes either
way), each targeting the second-declared overload so the pins fail
under declaration order.
pbednarcik
6fc3453db3