Fixes #3799: manual forks were mis-grouped under compression-continuation lineage in the sidebar, collapsing a forked session under the wrong root. Explicit manual forks are now treated as lineage boundaries while enriched child-session rows stay independently visible. Also hardens the background index-rebuild thread to be active-session-dir-safe (pins + re-checks its target under the rebuild lock) so a concurrent profile/session-dir switch can't make a rebuild write the wrong directory's index.