What is the collapsed single-leaf rule in Kaspa's lane commitment?

Kaspa's lane commitment collapses any subtree that contains exactly one active key: instead of carrying empty sibling hashes all the way down to depth 256, the subtree commits directly to that lone key and its leaf hash. In a standard Sparse Merkle Tree, a single key buried deep in the tree would still require a proof that includes an empty hash at every level — up to 256 hashes just to prove one leaf exists. The collapsed rule skips all of that. Because lane keys are hash-distributed and the subnetwork rules restrict the preimage space, this gives Kaspa the same practical proof-size benefit that Patricia-style compression would offer, without the added complexity of compressed shared paths.