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/IPlayEventSink.cs

@@ -5,12 +5,30 @@ namespace DeepDrftPublic.Client.Services;
 /// <summary>
 /// The emit seam for the <see cref="PlayTracker"/> (Phase 16 §2.1). The tracker owns the session
 /// lifecycle, the engagement floor, and the bucket classification but knows nothing about transport —
-/// it hands a finished classification to a sink. The production sink fires a <c>sendBeacon</c> POST to
-/// <c>api/event/play</c>; tests substitute a fake sink to assert floor and bucket behaviour with no
-/// JS interop. This keeps the tracker's logic testable behind one seam, as the spec calls for.
+/// it hands a finished classification to a sink, choosing only which arm fits the close that triggered
+/// it. Two arms exist because the close paths differ in whether the page survives the call (telemetry
+/// transport-resilience):
+/// <list type="bullet">
+/// <item><see cref="EmitPlayAsync"/> — normal closes (organic end / track-switch / stop), where the page
+/// stays alive, go over a first-party <c>HttpClient</c> POST to <c>api/event/play</c>. A first-party
+/// fetch is not name-matched by tracking/fingerprinting heuristics the way a <c>sendBeacon</c> module is.</item>
+/// <item><see cref="EmitPlayOnUnload"/> — the page-unload edge (pagehide / visibility→hidden), where an
+/// awaited fetch would be cancelled, still goes over <c>navigator.sendBeacon</c>.</item>
+/// </list>
+/// Tests substitute a fake sink to assert floor and bucket behaviour with no transport.
 /// </summary>
 public interface IPlayEventSink
 {
-    /// <summary>Emit one recorded play. Called at most once per session, only when the floor is crossed.</summary>
-    void EmitPlay(string trackEntryKey, PlayBucket bucket);
+    /// <summary>
+    /// Emit one recorded play over the first-party fetch transport (normal close: end / switch / stop).
+    /// Called at most once per session, only when the floor is crossed. Awaitable but safe to
+    /// fire-and-forget on a live page; never throws.
+    /// </summary>
+    Task EmitPlayAsync(string trackEntryKey, PlayBucket bucket);
+
+    /// <summary>
+    /// Emit one recorded play over <c>sendBeacon</c> for the page-unload edge, where an awaited fetch
+    /// would be cancelled as the page freezes. Synchronous and fire-and-forget; never throws.
+    /// </summary>
+    void EmitPlayOnUnload(string trackEntryKey, PlayBucket bucket);
 }