/// Construct a `{{ struct_name }}` from Dart callback closures.
/// FRB synthesises a Dart-callable function type for each closure parameter,
/// which is the whole point of taking them as `impl Fn(...) -> DartFnFuture<R>`
/// parameters rather than storing them as `Box<dyn Fn(...)>` fields on an opaque
/// struct (FRB v2 silently drops factories that return opaque structs whose fields
/// it cannot bridge). The returned wrapper holds an `Arc<dyn Trait + Send + Sync>`
/// whose backing object carries the supplied callbacks privately.