fix(driver-modbus): resolve High code-review finding (Driver.Modbus-001)

_lastPublishedByRef was a plain Dictionary<string, object> mutated inside
ShouldPublish, which runs on the PollGroupEngine onChange callback. The engine
runs one background Task per subscription, so a driver with two or more
subscriptions invokes ShouldPublish concurrently on separate threads. Concurrent
TryGetValue/indexer writes on a non-thread-safe Dictionary can corrupt internal
state, drop entries, or throw, crashing the poll loop.

Switch _lastPublishedByRef to ConcurrentDictionary<string, object>; its
TryGetValue and indexer-set operations are individually thread-safe, so the
deadband cache is now correct under concurrent multi-subscription publishing,
consistent with the lock-guarded sibling cache _lastWrittenByRef.

Add an xUnit + Shouldly regression test that runs 24 deadband-configured
single-tag subscriptions concurrently and asserts the poll loop survives without
faulting.

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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriver.cs

@@ -1,4 +1,5 @@
 using System.Buffers.Binary;
+using System.Collections.Concurrent;
 using System.Text.Json;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
@@ -89,7 +90,11 @@ public sealed class ModbusDriver
     // Last-published value per tag, keyed by FullReference. Used by ShouldPublish to apply
     // the deadband filter. Stored as object so all numeric types share one map; the comparison
     // does a typed cast inside.
-    private readonly Dictionary<string, object> _lastPublishedByRef = new(StringComparer.OrdinalIgnoreCase);
+    // Driver.Modbus-001: ShouldPublish runs on the PollGroupEngine onChange callback, which
+    // executes on one background Task per subscription — so a multi-subscription driver mutates
+    // this map concurrently from several threads. A plain Dictionary corrupts under concurrent
+    // writes; ConcurrentDictionary makes every TryGetValue / indexer write thread-safe.
+    private readonly ConcurrentDictionary<string, object> _lastPublishedByRef = new(StringComparer.OrdinalIgnoreCase);
 
     // Last-written value per tag for the WriteOnChangeOnly suppression. Invalidated by reads
     // that return a different value (so an HMI-side change doesn't get masked).