perf(drivers): fixed-rate poll pacing with overrun counter; non-blocking unsubscribe

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbCip/AbCipDriver.cs

@@ -448,10 +448,11 @@ public sealed class AbCipDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         Task.FromResult(_poll.Subscribe(fullReferences, publishingInterval));
 
     /// <inheritdoc />
-    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
-        _poll.Unsubscribe(handle);
-        return Task.CompletedTask;
+        // Async teardown: the engine drains the in-flight poll without blocking the caller's
+        // thread — this runs inside a DriverInstanceActor mailbox turn.
+        await _poll.UnsubscribeAsync(handle).ConfigureAwait(false);
     }
 
     // ---- IAlarmSource (ALMD projection) ----

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

@@ -489,10 +489,11 @@ public sealed class AbLegacyDriver : IDriver, IReadable, IWritable, ITagDiscover
         Task.FromResult(_poll.Subscribe(fullReferences, publishingInterval));
 
     /// <inheritdoc />
-    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
-        _poll.Unsubscribe(handle);
-        return Task.CompletedTask;
+        // Async teardown: the engine drains the in-flight poll without blocking the caller's
+        // thread — this runs inside a DriverInstanceActor mailbox turn.
+        await _poll.UnsubscribeAsync(handle).ConfigureAwait(false);
     }
 
     // ---- IHostConnectivityProbe ----

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/FocasDriver.cs

@@ -838,10 +838,11 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         Task.FromResult(_poll.Subscribe(fullReferences, publishingInterval));
 
     /// <inheritdoc />
-    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
-        _poll.Unsubscribe(handle);
-        return Task.CompletedTask;
+        // Async teardown: the engine drains the in-flight poll without blocking the caller's
+        // thread — this runs inside a DriverInstanceActor mailbox turn.
+        await _poll.UnsubscribeAsync(handle).ConfigureAwait(false);
     }
 
     // ---- IHostConnectivityProbe ----

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

@@ -1341,13 +1341,14 @@ public sealed class ModbusDriver
         Task.FromResult(_poll.Subscribe(fullReferences, publishingInterval));
 
     /// <inheritdoc />
-    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
-        _poll.Unsubscribe(handle);
+        // Async teardown: the engine drains the in-flight poll without blocking the caller's
+        // thread — this runs inside a DriverInstanceActor mailbox turn.
+        await _poll.UnsubscribeAsync(handle).ConfigureAwait(false);
         // Drop any partially-collected cycle buffer so a torn-down subscription leaks nothing.
-        // Unsubscribe awaits the loop task, so no onChange can re-add after this point.
+        // The teardown above awaits the loop task, so no onChange can re-add after this point.
         _cycleBatchItems.TryRemove(handle, out _);
-        return Task.CompletedTask;
     }
 
     // ---- IHostConnectivityProbe ----

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -1732,10 +1732,11 @@ public sealed class S7Driver
         => Task.FromResult(_poll.Subscribe(fullReferences, publishingInterval));
 
     /// <inheritdoc />
-    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
-        _poll.Unsubscribe(handle);
-        return Task.CompletedTask;
+        // Async teardown: the engine drains the in-flight poll without blocking the caller's
+        // thread — this runs inside a DriverInstanceActor mailbox turn.
+        await _poll.UnsubscribeAsync(handle).ConfigureAwait(false);
     }
 
     /// <summary>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Sql/SqlDriver.cs

@@ -570,10 +570,11 @@ public sealed class SqlDriver
     }
 
     /// <inheritdoc />
-    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
-        _poll.Unsubscribe(handle);
-        return Task.CompletedTask;
+        // Async teardown: the engine drains the in-flight poll without blocking the caller's
+        // thread — this runs inside a DriverInstanceActor mailbox turn.
+        await _poll.UnsubscribeAsync(handle).ConfigureAwait(false);
     }
 
     // ---- IHostConnectivityProbe ----

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.TwinCAT/TwinCATDriver.cs

@@ -576,17 +576,18 @@ public sealed class TwinCATDriver : IDriver, IReadable, IWritable, ITagDiscovery
     }
 
     /// <inheritdoc />
-    public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
+    public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
         if (handle is NativeSubscriptionHandle native && _nativeSubs.TryRemove(native.Id, out var sub))
         {
             // NativeRegistration.Dispose removes itself from its device registry (so a later replay
             // won't touch it) and disposes its live handle.
             foreach (var r in sub.Registrations) { try { r.Dispose(); } catch { } }
-            return Task.CompletedTask;
+            return;
         }
-        _poll.Unsubscribe(handle);
-        return Task.CompletedTask;
+        // Async teardown: the engine drains the in-flight poll without blocking the caller's
+        // thread — this runs inside a DriverInstanceActor mailbox turn.
+        await _poll.UnsubscribeAsync(handle).ConfigureAwait(false);
     }
 
     private sealed record NativeSubscriptionHandle(long Id) : ISubscriptionHandle