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.

DeepDrftPublic.Client/Services/PlayTracker.cs

@@ -88,8 +88,15 @@ public sealed class PlayTracker
     /// nothing is sent (it was a preview/skip, §1d). Idempotent and safe to call when no session is open —
     /// organic end, track-switch, stop, dispose, and the unload beacon may all race to close, and only the
     /// first call emits.
+    ///
+    /// <para><paramref name="viaUnload"/> selects the transport, not the classification (telemetry
+    /// transport-resilience). The default (false) is the normal close (organic end / track-switch / stop):
+    /// the page is alive, so the event goes over the first-party fetch arm. The unload handler passes true
+    /// so the rare tab-close mid-play uses <c>sendBeacon</c>, the only transport that survives the freeze.
+    /// The fetch arm is fire-and-forget here because the close paths are sync-shaped (a void JS callback,
+    /// or a teardown we must not block on a telemetry POST) — on a live page the task still completes.</para>
     /// </summary>
-    public void Close()
+    public void Close(bool viaUnload = false)
     {
         if (!HasOpenSession)
         {
@@ -112,7 +119,11 @@ public sealed class PlayTracker
         if (!CrossesFloor(_highWater, duration))
             return;
 
-        _sink.EmitPlay(key, Classify(fraction));
+        var bucket = Classify(fraction);
+        if (viaUnload)
+            _sink.EmitPlayOnUnload(key, bucket);
+        else
+            _ = _sink.EmitPlayAsync(key, bucket);
     }
 
     // The floor is the SMALLER of the absolute-seconds wall and the percentage of duration (§1d / D2).