Receiver Supports No Sender

A network protocol receiver carries decode logic for an optional feature that no sender in the project's own code ever produces. The codepath is unreachable from the application's own clients and therefore unexercised by its tests, fuzzers, and reviews. It is fully reachable, and exclusively exercised, by attacker traffic that sets the trigger field on the wire. Speculative receiver features added "in case we want it later" become attack surface when "later" means never.