fix(telemetry): first-party fetch for play/share, beacon only on unload

Route normal play closes (end/switch/stop) and all shares through a same-origin
HttpClient POST so privacy-hardened browsers stop blocking them; keep sendBeacon
for the tab-unload edge. Rename the JS module off telemetry/beacon to session/
lifecycle so the retained fallback isn't name-matched. No new data or identifiers.

DeepDrftTests/PlayTrackerTests.cs

@@ -13,11 +13,27 @@ namespace DeepDrftTests;
 [TestFixture]
 public class PlayTrackerTests
 {
-    // Captures emitted plays so assertions read the (key, bucket) the tracker classified.
+    // Captures emitted plays so assertions read the (key, bucket) the tracker classified. The two arms are
+    // captured separately so a test can assert which transport a given close selected (fetch vs unload).
+    // Emitted folds both arms for the floor/bucket assertions that don't care about transport.
     private sealed class FakeSink : IPlayEventSink
     {
         public List<(string Key, PlayBucket Bucket)> Emitted { get; } = new();
-        public void EmitPlay(string trackEntryKey, PlayBucket bucket) => Emitted.Add((trackEntryKey, bucket));
+        public List<(string Key, PlayBucket Bucket)> FetchEmitted { get; } = new();
+        public List<(string Key, PlayBucket Bucket)> UnloadEmitted { get; } = new();
+
+        public Task EmitPlayAsync(string trackEntryKey, PlayBucket bucket)
+        {
+            FetchEmitted.Add((trackEntryKey, bucket));
+            Emitted.Add((trackEntryKey, bucket));
+            return Task.CompletedTask;
+        }
+
+        public void EmitPlayOnUnload(string trackEntryKey, PlayBucket bucket)
+        {
+            UnloadEmitted.Add((trackEntryKey, bucket));
+            Emitted.Add((trackEntryKey, bucket));
+        }
     }
 
     private FakeSink _sink = null!;
@@ -202,4 +218,34 @@ public class PlayTrackerTests
         PlaySession("t", duration: 100, highWater: 95);
         Assert.That(_sink.Emitted, Has.Count.EqualTo(2));
     }
+
+    // --- Transport-arm selection (telemetry transport-resilience) ---
+
+    // A normal close (organic end / track-switch / stop) emits over the first-party fetch arm — the page
+    // is alive, so the awaitable HttpClient POST is the heuristic-safe transport.
+    [Test]
+    public void Close_NormalClose_EmitsOverFetchArm()
+    {
+        _tracker.OnPlaybackStarted("t");
+        _tracker.SetDuration(100);
+        _tracker.OnProgress(95);
+        _tracker.Close();   // viaUnload defaults to false
+
+        Assert.That(_sink.FetchEmitted, Has.Count.EqualTo(1));
+        Assert.That(_sink.UnloadEmitted, Is.Empty);
+    }
+
+    // The page-unload close emits over the sendBeacon arm — an awaited fetch would be cancelled as the
+    // page freezes, so this rare edge keeps the beacon.
+    [Test]
+    public void Close_ViaUnload_EmitsOverBeaconArm()
+    {
+        _tracker.OnPlaybackStarted("t");
+        _tracker.SetDuration(100);
+        _tracker.OnProgress(95);
+        _tracker.Close(viaUnload: true);
+
+        Assert.That(_sink.UnloadEmitted, Has.Count.EqualTo(1));
+        Assert.That(_sink.FetchEmitted, Is.Empty);
+    }
 }