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/AudioPlayerService.cs

@@ -260,6 +260,9 @@ public abstract class AudioPlayerService : IPlayerService, IAsyncDisposable
     private async Task OnProgressCallback(double currentTime)
     {
         CurrentTime = currentTime;
+        // Telemetry hook (Phase 16 §2.1): a subclass advances the play-session high-water mark here, on
+        // the same throttled tick the UI already consumes. Base implementation is a no-op.
+        OnProgressTick(currentTime);
         await NotifyStateChanged();
     }
 
@@ -270,6 +273,10 @@ public abstract class AudioPlayerService : IPlayerService, IAsyncDisposable
         IsLoaded = false;
         CurrentTime = 0;
         Duration = null;
+        // Telemetry hook: organic end closes the play session (the bucket reflects how far they got)
+        // BEFORE the state notification and TrackEnded fan-out, so the session that just ended is the
+        // one recorded — not whatever a queue auto-advance opens next. Base implementation is a no-op.
+        OnPlaybackEnded();
         await NotifyStateChanged();
 
         // Fire AFTER the state notification so any queue orchestrator that advances on this
@@ -279,6 +286,18 @@ public abstract class AudioPlayerService : IPlayerService, IAsyncDisposable
         TrackEnded?.Invoke();
     }
 
+    /// <summary>
+    /// Telemetry seam (Phase 16): called on each progress tick with the current playback position. The
+    /// streaming subclass overrides this to advance the play-session high-water mark. No-op in the base.
+    /// </summary>
+    protected virtual void OnProgressTick(double currentTime) { }
+
+    /// <summary>
+    /// Telemetry seam (Phase 16): called on organic end-of-stream, before <see cref="TrackEnded"/> fires.
+    /// The streaming subclass overrides this to close the play session. No-op in the base.
+    /// </summary>
+    protected virtual void OnPlaybackEnded() { }
+
 
     protected async Task EnsureInitializedAsync()
     {