* fix(filer.sync): guard batched events against nil EventNotification
The server folds a backlog into one response: the first event in the
top-level fields, the rest in resp.Events, and the pipelined sender can
drain an idle heartbeat (nil EventNotification) into that tail. Only the
envelope got the freshness-signal guard, so a batched heartbeat reached
AddSyncJob and nil-derefed in IsEmpty while replaying a backlog buffered
during a peer outage.
Route every event, envelope and batched, through one handler that sends
freshness signals (nil heartbeat, empty marker) to OnIdleHeartbeat.
* fix(filer): guard MetaAggregator batched events against nil EventNotification
The peer subscription's envelope is nil-guarded but its batched tail was
not. The aggregator doesn't enable idle heartbeats today, so the server
can't fold a nil EventNotification into the batch yet, but make the two
loops consistent so it can't nil-deref if that changes.
On a read-only watched path the idle heartbeat keeps sync_offset fresh,
but a busy source filer still emits a MaxUnsyncedEvents marker after many
filtered events. The marker has a non-nil but empty EventNotification, so
the client routed it to the event path, where it advanced no real
watermark yet drove offsetFunc to republish the stale processed
watermark — regressing the gauge between heartbeats and spiking the
derived lag every time a filtered-event burst landed.
Route the empty marker through OnIdleHeartbeat like the idle heartbeat so
its fresh timestamp keeps the gauge current; it still advances the
in-stream resume cursor.
Chris Lu