ULOG tickets: append-log & store durability

Discrepancies in dog/ULOG.* — the append-only <ts>\t<verb>\t<uri> event log (worktree .be / store wtlog) and the refs/.refs.idx tip files built on it. Durability and crash-safety of these logs. Each entry is one ticket with its live state; it drops off once the ticket closes. The method is Issues.

Active tickets

(none — ULOG-002 landed dog df3b5885+abc e8d7c59d; ULOG-003 landed dog be89684d+beagle e5071f32.)

The initial-NUL trigger (what writes the leading NUL) remains unpinned (see ULOG-001).