Exercise 1: add a third stop

Question: make h1 -> h3 traffic visit leaf1's own uN explicitly. What's the address, and what does leaf1 do?

Answer: fcbb:bb00:1:100:3:fe00::   (block fcbb:bb00, then leaf1 = 1, spine1 = 100, leaf3 = 3, decap = fe00)

leaf1 sees its own uN (fcbb:bb00:1::/48, "End flavors next-csid"), shifts its 16 bits out, and the address
becomes fcbb:bb00:100:3:fe00::. Now it's an ordinary packet for spine1, forwarded on leaf1's static /48.
spine1 punches 100, leaf3 decaps (End.DX4). Three uSIDs in one 128-bit address, still ONE SRH segment.
With F3216 (32-bit block, 16-bit IDs) one address holds up to 6 uSIDs.

Reproduce:  sudo ./solutions/exercise1/run.sh
Kit run (containerlab, 10/19/2026):
  5 packets transmitted, 5 received, 0% packet loss
  dst leaving h1 (what h1 wrote):      fcbb:bb00:1:100:3:fe00::
  dst leaving leaf1 towards spine1:    fcbb:bb00:100:3:fe00::
