test(engine): cover RebuildFinder failure + success paths

Three new tests around the finder rebuild:

  - Failure path: CreateFinder throws on the 2nd call. RebuildFinder
    returns false, keeps the original (non-disposed) finder, and a
    follow-up PollOnce still reads sources. Pre-fix this regressed:
    the incumbent was disposed before the throw, so PollOnce hit a
    disposed handle (now an ObjectDisposedException from the fake).

  - Success path: RebuildFinder returns true, builds exactly one
    replacement, and re-emits all currently-visible sources as Added
    (seen-set cleared).

  - Failure path preserves the seen-set: no spurious Added re-fires
    for sources that were already known.

src/tests/Dragon-ISO.Engine.Tests/Discovery/NdiDiscoveryServiceTests.cs

@@ -67,6 +67,86 @@ public class NdiDiscoveryServiceTests
         return list;
     }
 
+    // ============================================================
+    // RebuildFinder — swap-then-dispose, failure keeps the old finder
+    // ============================================================
+
+    [Fact]
+    public void RebuildFinder_WhenCreateFails_KeepsExistingFinder_AndKeepsPolling()
+    {
+        // First finder (construction) succeeds; the rebuild's CreateFinder throws.
+        var interop = new FakeNdiInterop
+        {
+            CreateFinderHook = ordinal =>
+            {
+                if (ordinal >= 2) throw new InvalidOperationException("runtime refused a new finder");
+            },
+        };
+        interop.Sources.Add("PC1 (Teams - Jane)");
+        var channel = Channel.CreateUnbounded<DiscoveryEvent>();
+        var svc = new NdiDiscoveryService(interop, channel.Writer, NullLogger<NdiDiscoveryService>.Instance);
+
+        // Prime the seen-set.
+        svc.PollOnce();
+        DrainChannel(channel.Reader);
+
+        // Rebuild fails — must return false and must NOT dispose the live finder.
+        svc.RebuildFinder("test: simulated failure").Should().BeFalse();
+        interop.FinderCreatedCount.Should().Be(2); // initial + the failed attempt
+
+        // The incumbent finder is still alive: PollOnce reads it without throwing.
+        // Pre-fix this threw ObjectDisposedException because the finder had been
+        // disposed before the failing CreateFinder call.
+        var poll = () => svc.PollOnce();
+        poll.Should().NotThrow();
+    }
+
+    [Fact]
+    public void RebuildFinder_FailureKeepsSeenSet_NoSpuriousReAdd()
+    {
+        var interop = new FakeNdiInterop
+        {
+            CreateFinderHook = ordinal =>
+            {
+                if (ordinal >= 2) throw new InvalidOperationException("boom");
+            },
+        };
+        interop.Sources.Add("PC1 (Teams - Jane)");
+        var channel = Channel.CreateUnbounded<DiscoveryEvent>();
+        var svc = new NdiDiscoveryService(interop, channel.Writer, NullLogger<NdiDiscoveryService>.Instance);
+
+        svc.PollOnce();
+        DrainChannel(channel.Reader); // consume the initial Added
+
+        svc.RebuildFinder("test: simulated failure").Should().BeFalse();
+        svc.PollOnce();
+
+        // Seen-set was preserved across the failed rebuild, so the already-known
+        // source must NOT re-fire as Added.
+        DrainChannel(channel.Reader).Should().BeEmpty();
+    }
+
+    [Fact]
+    public void RebuildFinder_OnSuccess_SwapsFinder_AndReEmitsKnownSources()
+    {
+        var interop = new FakeNdiInterop();
+        interop.Sources.Add("PC1 (Teams - Jane)");
+        var channel = Channel.CreateUnbounded<DiscoveryEvent>();
+        var svc = new NdiDiscoveryService(interop, channel.Writer, NullLogger<NdiDiscoveryService>.Instance);
+
+        svc.PollOnce();
+        DrainChannel(channel.Reader); // consume initial Added
+
+        svc.RebuildFinder("test: success").Should().BeTrue();
+        interop.FinderCreatedCount.Should().Be(2);
+
+        // Seen-set was cleared on success, so the still-visible source re-fires.
+        svc.PollOnce();
+        DrainChannel(channel.Reader).OfType<DiscoveryEvent.Added>()
+            .Select(a => a.Source.FullName)
+            .Should().BeEquivalentTo(new[] { "PC1 (Teams - Jane)" });
+    }
+
     // ============================================================
     // ShouldAutoRebuild — pure function gating the auto-heal path
     // ============================================================