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/BeaconPlayEventSink.cs

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+using System.Text.Json;
+using DeepDrftModels.DTOs;
+using DeepDrftModels.Enums;
+using Microsoft.AspNetCore.Components;
+
+namespace DeepDrftPublic.Client.Services;
+
+/// <summary>
+/// Production <see cref="IPlayEventSink"/> (Phase 16 §2.2): serializes the play classification and fires
+/// it via <c>navigator.sendBeacon</c> to the proxied <c>api/event/play</c> route. Fire-and-forget by
+/// design — <see cref="IPlayEventSink.EmitPlay"/> is synchronous (it is called from the player's close
+/// path and the unload handler, neither of which can await), so the beacon is dispatched without
+/// awaiting and its failure is irrelevant. No <c>anonId</c> is sent in wave 16.1.
+/// </summary>
+public sealed class BeaconPlayEventSink : IPlayEventSink
+{
+    private readonly BeaconInterop _beacon;
+    private readonly string _playUrl;
+
+    public BeaconPlayEventSink(BeaconInterop beacon, NavigationManager navigation)
+    {
+        _beacon = beacon;
+        // The WASM client posts to its own host, which proxies to DeepDrftAPI. BaseUri carries a
+        // trailing slash; the route does not lead with one.
+        _playUrl = $"{navigation.BaseUri}api/event/play";
+    }
+
+    public void EmitPlay(string trackEntryKey, PlayBucket bucket)
+    {
+        var json = JsonSerializer.Serialize(new PlayEventDto
+        {
+            TrackEntryKey = trackEntryKey,
+            Bucket = bucket,
+        });
+
+        // Fire-and-forget: do not await. The beacon survives unload; the C# task may not, and we do not
+        // act on the result either way.
+        _ = _beacon.SendAsync(_playUrl, json);
+    }
+}