PR 6.2 — Bounded EventPump channel + drop-newest metrics

Decouples the gw stream-read loop from the listener-fanout loop with a
bounded Channel<MxEvent> (default capacity 50_000) sitting between them.
When a slow listener fills the channel, the producer's TryWrite returns
false and we count the drop rather than back-pressuring the gw stream.

Three counters on the ZB.MOM.WW.OtOpcUa.Driver.Galaxy meter expose the
pressure curve before it manifests as user-visible loss:

- galaxy.events.received  — MxEvents read from StreamEvents
- galaxy.events.dispatched — MxEvents that made it through to OnDataChange
- galaxy.events.dropped   — MxEvents discarded because the channel was full

Each measurement carries a galaxy.client tag so multi-driver hosts can
split by source. The driver wires _options.MxAccess.ClientName into the
new EventPump constructor parameter.

Tests: drop-newest under pressure, capacity validation, and per-pump
measurement filtering (xUnit can run other pump tests in parallel and
their measurements land on the same listener — the test filters to its
own client name).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs

@@ -511,7 +511,9 @@ public sealed class GalaxyDriver
         lock (_pumpLock)
         {
             if (_eventPump is not null) return _eventPump;
-            _eventPump = new EventPump(_subscriber!, _subscriptions, _logger);
+            _eventPump = new EventPump(
+                _subscriber!, _subscriptions, _logger,
+                clientName: _options.MxAccess.ClientName);
             _eventPump.OnDataChange += OnPumpDataChange;
             _eventPump.Start();
             return _eventPump;