Stall-without-pop is only safe when a FIFO carries a single destination

A FIFO that mixes several destinations cannot hold its head when one target is paused or paced, because all packets behind it, bound for unblocked targets, are stuck too. Holding the head is acceptable only for a single-target FIFO. Per-destination FIFOs or a bypass/skip structure are the fixes, at the cost of more storage or write-side steering logic.