InstanceActor._tagPathToAttribute was a Dictionary<string,string> — one tag
path mapped to a single attribute. When two attributes reference the same PLC
node (e.g. two composed cooling-tank modules both reading ns=3;s=Tank.Level,
or a pump's TempSensor and AlarmSensor both reading ns=3;s=Sensor.Reading),
SubscribeToDcl's map assignment overwrote, so only the last-registered
attribute ever received values — the rest stayed permanently Uncertain.
The map is now Dictionary<string,List<string>>; HandleTagValueUpdate fans each
update out to every attribute referencing the tag path, and each distinct tag
path is still subscribed only once per connection.
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