Amend spec of dyn and some

The problem currently:
* `dyn InterfaceType varDecl` must have a `dyn interface` type
* if we are `dyn interface` we ban `opaque` `non-copyable` `unsized`
* Therefore we have no solution to not break user-code using interfaces in 2025 as variables/parameters since fixing 1 problem makes the other an issue (especially problematic with auto-diff interfaces).