feat(home-dialog): per-session suppression tracker

Singleton keyed by ShortUdid; lock on per-viewer set to avoid
cross-viewer contention. Process lifetime — restart re-fires.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.UnitTests/Services/HomeDialogSessionTrackerTests.cs

@@ -0,0 +1,53 @@
+using SVSim.EmulatedEntrypoint.Services;
+
+namespace SVSim.UnitTests.Services;
+
+public class HomeDialogSessionTrackerTests
+{
+    [Test]
+    public void TryReserve_returns_true_first_time_and_false_on_repeat_for_same_viewer_and_dialog()
+    {
+        var tracker = new HomeDialogSessionTracker();
+
+        Assert.That(tracker.TryReserve(viewerShortUdid: 100, dialogId: 1), Is.True);
+        Assert.That(tracker.TryReserve(viewerShortUdid: 100, dialogId: 1), Is.False);
+    }
+
+    [Test]
+    public void TryReserve_is_independent_across_viewers()
+    {
+        var tracker = new HomeDialogSessionTracker();
+
+        Assert.That(tracker.TryReserve(100, 1), Is.True);
+        Assert.That(tracker.TryReserve(200, 1), Is.True, "viewer 200 must see the dialog even though viewer 100 already did");
+    }
+
+    [Test]
+    public void TryReserve_is_independent_across_dialog_ids_for_one_viewer()
+    {
+        var tracker = new HomeDialogSessionTracker();
+
+        Assert.That(tracker.TryReserve(100, 1), Is.True);
+        Assert.That(tracker.TryReserve(100, 2), Is.True);
+        Assert.That(tracker.TryReserve(100, 1), Is.False);
+        Assert.That(tracker.TryReserve(100, 2), Is.False);
+    }
+
+    [Test]
+    public void TryReserve_is_thread_safe_under_concurrent_calls_for_one_viewer()
+    {
+        var tracker = new HomeDialogSessionTracker();
+        const int dialogId = 42;
+        const int parallelism = 200;
+        int trueCount = 0;
+
+        Parallel.For(0, parallelism, _ =>
+        {
+            if (tracker.TryReserve(viewerShortUdid: 1, dialogId: dialogId))
+                Interlocked.Increment(ref trueCount);
+        });
+
+        Assert.That(trueCount, Is.EqualTo(1),
+            "Exactly one thread must win the reservation; the rest must observe false.");
+    }
+}