Optional causal embedding

No physical causal geometry is needed for the channel identity. If a realization is assigned locations, a causal embedding can be added as a separate contract.

Let the complete finite dependency graph include every actual quantum input, classical control, preparation, routing operation, and retained record. Assign each event a location $p$ in a set with a supplied transitive causal relation $⪯$. Require every dependency edge $p\to q$ to satisfy $p⪯q$.