fold Transport's alloc/qout into the .quic Link payload

open   by a73x

Labels: backlog, refactor, quic

From M15 (Task 6 review). Transport.alloc is `undefined` and qout unused for .fd/.pipe links — both are set and read only on the .quic path. Folding them into the .quic Link payload deletes the struct's one remaining `undefined` and makes "only quic has an outbound queue" structural. Compiler already confines the readers, so this is future-proofing; blast radius is the six link switches in client.zig. See decisions.md M15.