/// Build an `I{{ trait_name }}` vtable for a concrete Zig type `T`.
///
/// `T` must implement every method of `{{ trait_name }}` as a plain Zig function.
/// Each slot is wrapped in a `callconv(.C)` thunk that casts `user_data`
/// back to `*T` and forwards the call.
///
/// # Usage
/// ```zig
/// const vtable = make_{{ snake }}_vtable(MyType, &my_instance);
/// _ = register_{{ snake }}("my-impl", vtable, &my_instance, &out_error);
/// ```