Start each session's claims window at its turn's start

Measuring claims from a session's own previous snapshot (ec692747) made sessions that take turns in one worktree claim each other's files: B's window since its last snapshot spanned A's whole turn. The window now starts at the newest shadow snapshot when the session's turn started (the base commit when there is none), and each of its own snapshots moves it forward. What another session snapshotted while this one was idle is out of its window; what this session wrote while another snapshotted mid-turn is still in it.