Type Alias EmbedIr 

pub type EmbedIr = (TyId, String);

The payload of TypeShape::Sum’s own embeds — a resolved embeds clause ([DECISION C], #1161): the source type paired with the target variant’s own tag name. A plain tuple, not a dedicated struct, mirroring IrPat::Variant’s own fields: Vec<(String, Box<IrPat>)> precedent for a two-part fact with no further structure.