#[compose]Expand description
Compose rejection families from a list of sources: Source imports every
case under its original variant name, Source as Prefix under a prefixed
one. Collisions between unprefixed sources require a local
#[compose(merge)] variant; use #[compose(merge(SourceA::Case, SourceB::OtherCase))] to select participants explicitly. Merged variants
own their metadata through an explicit rejection code or payload
delegation. Every source gets a total lift.