Summary

Follow-ups from #3565: two scoping bugs that only became reachable now that a command can declare a real concurrency scope.

Fixed

The MongoDB event sequence storage narrows the tail sequence number by an actual event source type, and stops narrowing on the Default/Unspecified sentinel. It had the check inverted, so asking for the tail of one event source type answered across every type, and asking for the tail of everything answered with only the events carrying the sentinel.
The optimistic concurrency strategy reads the expected tail with the same narrowing it puts on the scope. It read the tail of the event source alone while the kernel validates through the scope's stream type, stream id and event source type, so the two could disagree — reporting conflicts between independent appends, or missing real ones.