Function slot_size_for_mtu 

pub fn slot_size_for_mtu(mtu: usize) -> u32

Pick a slot size for a face that needs to carry NDN Data whose content can be up to mtu bytes. Rounds up to the next multiple of 64 bytes so the per-slot stride stays cache-line aligned. Does not clamp to a floor — an explicit mtu = 8800 gets a ~25 KiB slot, not the 272 KiB default. If no mtu is specified, the caller should use DEFAULT_SLOT_SIZE directly.