fix(focas): fail-fast at init on unimplemented backend (operator footgun)

Add IFocasClientFactory.EnsureUsable() — a config-time probe called by
FocasDriver.InitializeAsync before any background loops start. The
UnimplementedFocasClientFactory throws NotSupportedException immediately
(faulting the driver at init), eliminating the footgun where a driver on
the 'unimplemented' backend appeared Healthy then failed every read/write/
subscribe silently. WireFocasClientFactory and FakeFocasClientFactory are
no-ops. Backstop Create() throw remains in place.

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

@@ -89,6 +89,11 @@ public sealed class FocasDriver : IDriver, IReadable, IWritable, ITagDiscovery,
         Volatile.Write(ref _health, new DriverHealth(DriverState.Initializing, null, null));
         try
         {
+            // Fail fast if the factory is a stub/unimplemented backend — the operator must
+            // see an actionable error at init rather than a phantom-Healthy driver that fails
+            // every read/write/subscribe silently (Driver.FOCAS-009).
+            _clientFactory.EnsureUsable();
+
             foreach (var device in _options.Devices)
             {
                 var addr = FocasHostAddress.TryParse(device.HostAddress)

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

@@ -282,6 +282,18 @@ public interface IFocasClientFactory
     /// <summary>Creates a new FOCAS client instance.</summary>
     /// <returns>A new IFocasClient instance.</returns>
     IFocasClient Create();
+
+    /// <summary>
+    ///     Config-time usability probe called by <see cref="FocasDriver.InitializeAsync"/> before
+    ///     any background loops start. Must NOT create a live wire client. Implementations that
+    ///     are permanently non-functional (e.g. <see cref="UnimplementedFocasClientFactory"/>)
+    ///     throw <see cref="NotSupportedException"/> here so the driver faults immediately at
+    ///     init rather than lazily on the first read.
+    /// </summary>
+    /// <exception cref="NotSupportedException">
+    ///     Thrown by stub/unimplemented backends to force a fail-fast init fault.
+    /// </exception>
+    void EnsureUsable();
 }
 
 /// <summary>
@@ -292,12 +304,22 @@ public interface IFocasClientFactory
 /// </summary>
 public sealed class UnimplementedFocasClientFactory : IFocasClientFactory
 {
+    private const string Message =
+        "FOCAS driver backend is 'unimplemented'. Switch to 'Backend: \"wire\"' in driver config " +
+        "once the CNC is provisioned — see docs/drivers/FOCAS.md.";
+
+    /// <summary>
+    ///     Config-time probe — throws immediately so <see cref="FocasDriver.InitializeAsync"/>
+    ///     faults the driver before any background loops start. This prevents the footgun where
+    ///     the driver appears Healthy at init but every read/write/subscribe fails.
+    /// </summary>
+    /// <exception cref="NotSupportedException">Always thrown.</exception>
+    public void EnsureUsable() => throw new NotSupportedException(Message);
+
     /// <summary>Creates a new client instance (always throws NotSupportedException).</summary>
     /// <returns>Never returns; always throws NotSupportedException.</returns>
     /// <exception cref="NotSupportedException">Always thrown to indicate backend is not yet provisioned.</exception>
-    public IFocasClient Create() => throw new NotSupportedException(
+    public IFocasClient Create() => throw new NotSupportedException(Message);
-        "FOCAS driver backend is 'unimplemented'. Switch to 'Backend: \"wire\"' in driver config " +
-        "once the CNC is provisioned — see docs/drivers/FOCAS.md.");
 }
 
 /// <summary>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS/Wire/WireFocasClient.cs

@@ -427,6 +427,12 @@ public sealed class WireFocasClientFactory : IFocasClientFactory
         _logger = logger;
     }
 
+    /// <summary>
+    ///     No-op usability probe — the wire backend is always usable at config time.
+    ///     Implements <see cref="IFocasClientFactory.EnsureUsable"/>.
+    /// </summary>
+    public void EnsureUsable() { }
+
     /// <summary>Creates a new WireFocasClient instance.</summary>
     /// <returns>A new IFocasClient implementation.</returns>
     public IFocasClient Create() => new WireFocasClient(_logger);

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FakeFocasClient.cs

@@ -169,6 +169,9 @@ internal sealed class FakeFocasClientFactory : IFocasClientFactory
     /// <summary>Gets or sets a customization function for creating clients.</summary>
     public Func<FakeFocasClient>? Customise { get; set; }
 
+    /// <summary>No-op usability probe — the fake factory is always usable.</summary>
+    public void EnsureUsable() { }
+
     /// <summary>Creates a fake FOCAS client.</summary>
     public IFocasClient Create()
     {

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Tests/FocasScaffoldingTests.cs

@@ -261,4 +261,36 @@ public sealed class FocasScaffoldingTests
         ex.Message.ShouldContain("wire");
         ex.Message.ShouldContain("docs/drivers/FOCAS.md");
     }
+
+    /// <summary>
+    ///     Verifies a driver configured with the unimplemented backend faults at
+    ///     InitializeAsync — not lazily on the first read. The operator must get an
+    ///     actionable error immediately rather than a phantom-Healthy driver that
+    ///     fails every read silently.
+    /// </summary>
+    [Fact]
+    public async Task Unimplemented_backend_faults_at_InitializeAsync_not_on_first_read()
+    {
+        var drv = new FocasDriver(
+            new FocasDriverOptions { Devices = [new FocasDeviceOptions("focas://10.0.0.5:8193")] },
+            "drv-unimpl",
+            clientFactory: new UnimplementedFocasClientFactory());
+
+        var ex = await Should.ThrowAsync<NotSupportedException>(
+            () => drv.InitializeAsync("{}", CancellationToken.None));
+
+        ex.Message.ShouldContain("unimplemented");
+        drv.GetHealth().State.ShouldBe(DriverState.Faulted);
+    }
+
+    /// <summary>
+    ///     Verifies WireFocasClientFactory.EnsureUsable is a no-op — the wire backend
+    ///     must still initialize clean (no false-positive faults from the probe).
+    /// </summary>
+    [Fact]
+    public void Wire_factory_EnsureUsable_does_not_throw()
+    {
+        var factory = new Wire.WireFocasClientFactory();
+        Should.NotThrow(() => factory.EnsureUsable());
+    }
 }