feat(phase-16): anonymous play & share telemetry substrate (wave 16.1)

Player-service play-session tracker (floor + 3-bucket classify), SharePopover share tracker with debounce, sendBeacon interop, proxied rate-limited POST api/event/{play,share}, append-only event logs + incremental play_counter with server-side release resolution. Migration authored, not applied. No anonId, no read surface.

DeepDrftPublic.Client/Services/IPlayEventSink.cs

@@ -0,0 +1,16 @@
+using DeepDrftModels.Enums;
+
+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.
+/// </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);
+}