Internal transmitter STOP parks in one slot per FIFO, causing head-of-line blocking

The internal transmitter block has a single parking register per FIFO. When the head packet targets a stopped destination it is parked, and the FIFO's read enables stay low until the latch clears, so packets queued behind it for unblocked targets are stalled too. Only the other FIFOs keep moving via round-robin. FIFOs 0 and 2 carry multiple targets and are most exposed; FIFOs 1 and 3 carry only one target each.