ui : memoize leaf walks in sibling map build
buildSiblingInfoMap resolves each sibling's leaf by walking the last-child
chain, once per sibling per message, so the walk repeats along the same
chains for every message in the conversation ( O(messages^2) on long
chats ). Memoize leaf resolution per build with path compression so each
edge is walked once.
Assisted-by: pi:zai-org/GLM-5.3
