How does Kaspa's inactivity shortcut speed up lane-activity proofs?

Kaspa's inactivity shortcut lets a prover skip across large stretches of block history in far fewer steps. Normally, confirming that a lane (a logical activity channel in the protocol) has been inactive would require checking every block in a span — one step per block, or O(N) steps for N blocks. KIP-0021 adds a committed pointer inside each block called an inactivity shortcut: it jumps back to a specific ancestor at a fixed threshold distance, so a prover can leap that distance at a time and complete the same proof in O(N/F) hops instead. For beginners, this matters because it keeps Kaspa's protocol lightweight even as the chain grows — long inactivity spans do not become proportionally expensive to prove.