How does Kaspa assign transactions to lanes in KIP-0021?

In KIP-0021, each accepted transaction is assigned to a lane based on its subnetwork ID — so `lane_id(tx) = tx.subnetwork_id`. A lane is essentially a named channel that groups transactions of the same subnetwork together; each valid subnetwork ID gets its own dedicated lane rather than being merged into a single shared bucket. This matters for beginners because it means Kaspa's lane system is built directly into the existing transaction structure, making it predictable and consistent across the network without requiring any new transaction fields.