fix(driver-ablegacy): resolve Medium code-review finding (Driver.AbLegacy-008)

Mark _health volatile. The record-reference assignment is atomic, but
without an acquire/release memory barrier GetHealth() on another thread
can observe a stale snapshot indefinitely. volatile enforces the barrier
at read and write sites without a lock.

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

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy/AbLegacyDriver.cs

@@ -17,7 +17,13 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
     private readonly PollGroupEngine _poll;
     private readonly Dictionary<string, DeviceState> _devices = new(StringComparer.OrdinalIgnoreCase);
     private readonly Dictionary<string, AbLegacyTagDefinition> _tagsByName = new(StringComparer.OrdinalIgnoreCase);
-    private DriverHealth _health = new(DriverState.Unknown, null, null);
+
+    // volatile: _health is read by GetHealth() on any thread while ReadAsync / WriteAsync /
+    // InitializeAsync write it from worker / poll threads. The record-reference assignment is
+    // atomic on all .NET platforms, but without a memory barrier a reader can see a stale
+    // snapshot indefinitely. volatile enforces acquire/release ordering so GetHealth() always
+    // observes the most recently written value.
+    private volatile DriverHealth _health = new(DriverState.Unknown, null, null);
 
     public event EventHandler<DataChangeEventArgs>? OnDataChange;
     public event EventHandler<HostStatusChangedEventArgs>? OnHostStatusChanged;