`LiveEventHistory::snapshot_since` resumed from the oldest retained event
whenever a consumer's cursor pointed before it, without signaling that the
in-between events had been evicted from the ring buffer. Cursor-based
consumers therefore assumed the returned batch was contiguous with their
cursor and silently lost events.
Add a `gap` flag to `LiveEventBatch`, set when the consumer's cursor is
older than the oldest retained sequence, so consumers can detect the loss
and trigger a full re-sync or alert. Add a regression test that fills and
evicts past the ring-buffer capacity and asserts a lagging cursor reports
`gap = true` (and that contiguous/caught-up cursors do not).
Refs rustfs/backlog#969
Zhengchao An