The portal began as a distribution problem and became a statement of values. The Base collection would mint to the owner for a holder airdrop, with eligibility captured as late as practical so people entering the original collection near the crossing would not be erased by an early snapshot.
Direct mapping kept the movement legible: Sonic token number n could correspond to Base token number n. Bridged holders had to remain eligible too, because movement between chains should not make a real holder invisible.
The portal was never a random public mint. It was a controlled passage designed to carry the existing swarm into a new contract without manufacturing a second history.