DataConnectionActor seeded a tag's initial value by Tell-ing TagValueReceived
from HandleSubscribe's background task, which runs BEFORE HandleSubscribeCompleted
registers the instance's tags in _subscriptionsByInstance. HandleTagValueReceived's
fan-out then found no subscriber and dropped the value. A tag that soon gets a
data-change notification recovers, but a STATIC tag (e.g. an idle MES field that
never changes) was left Uncertain forever — the dropped seed was its only value.
Seeds now ride back on SubscribeCompleted and are delivered after registration,
reusing HandleTagValueReceived's generation guard, fan-out and quality accounting.
+1 regression test (DCL026).
Joseph Doherty