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.
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