test(engine): teach FakeNdiInterop to model finder failure + use-after-dispose

Additive, backwards-compatible test-double extensions so the discovery
rebuild failure path is observable:

  - CreateFinderHook: optional callback invoked on every CreateFinder,
    letting a test throw on the Nth call to simulate a runtime that
    refuses to build a replacement finder.
  - FinderCreatedCount: how many finders were built.
  - The fake find handle now flips a Disposed flag, and GetCurrentSources
    throws ObjectDisposedException if asked to read a disposed finder —
    so a regression that polls a retired finder fails loudly instead of
    silently passing (the fake previously ignored the handle entirely).

Default behavior is unchanged when CreateFinderHook is null.

src/tests/Dragon-ISO.Engine.Tests/Fakes/FakeNdiInterop.cs

@@ -24,12 +24,35 @@ public sealed class FakeNdiInterop : INdiInterop
     /// <summary>Per-output <c>groups</c> string seen by <see cref="CreateSender"/>; null = default Public.</summary>
     public Dictionary<string, string?> SenderGroups { get; } = new();
 
+    /// <summary>
+    /// Number of finders created so far (initial construction + every rebuild).
+    /// Lets tests assert how many times the discovery service tried to rebuild.
+    /// </summary>
+    public int FinderCreatedCount { get; private set; }
+
+    /// <summary>
+    /// Optional hook invoked at the top of every <see cref="CreateFinder"/> call,
+    /// receiving the 1-based creation ordinal. Throw from here to simulate a runtime
+    /// that refuses to build a finder (e.g. the rebuild-failure path); return normally
+    /// to allow creation. Null (default) = always succeed.
+    /// </summary>
+    public Action<int>? CreateFinderHook { get; set; }
+
     public NdiFindHandle CreateFinder(string? groups = null)
     {
+        FinderCreatedCount++;
+        CreateFinderHook?.Invoke(FinderCreatedCount);
         LastFinderGroups = groups;
         return new FakeFindHandle();
     }
-    public IReadOnlyList<string> GetCurrentSources(NdiFindHandle finder) => Sources.ToArray();
+
+    public IReadOnlyList<string> GetCurrentSources(NdiFindHandle finder)
+    {
+        if (finder is FakeFindHandle { Disposed: true })
+            throw new ObjectDisposedException(nameof(FakeFindHandle),
+                "GetCurrentSources called on a finder that was already disposed.");
+        return Sources.ToArray();
+    }
 
     public NdiReceiverHandle CreateReceiver(string sourceFullName)
     {
@@ -72,7 +95,8 @@ public sealed class FakeNdiInterop : INdiInterop
 
     private sealed class FakeFindHandle : NdiFindHandle
     {
-        public override void Dispose() { }
+        public bool Disposed { get; private set; }
+        public override void Dispose() => Disposed = true;
     }
 
     private sealed class FakeReceiverHandle : NdiReceiverHandle