Unknown-destination packet left at FIFO head blocks the queue forever

If an arbiter skips a FIFO head on a failed destination lookup without asserting read_en, that packet never leaves and every packet behind it is head-of-line blocked permanently. The fix is to pop and discard it (read_en=1 while staying in IDLE and advancing the FIFO round-robin pointer), ideally with a one-cycle drop pulse so upstream misconfiguration is visible. Check for the same pattern in every FIFO arm, since the fix must be applied symmetrically.