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.

DeepDrftModels/Enums/PlayBucket.cs

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+using System.Text.Json.Serialization;
+
+namespace DeepDrftModels.Enums;
+
+/// <summary>
+/// Completion bucket for a recorded play (Phase 16 §1a / D1). The three buckets are exhaustive and
+/// non-overlapping, classified by the high-water playback fraction reached before the session closed:
+/// <c>Partial</c> [0, 30%), <c>Sampled</c> [30%, 80%], <c>Complete</c> (80%, 100%]. The headline
+/// "Plays" figure is the sum of all three — every started listen that crosses the engagement floor
+/// is a play; the buckets are the texture beneath it.
+///
+/// Serialized as its string name on the wire — the converter on the type makes the
+/// client to proxy to API JSON contract string-based regardless of host serializer config.
+/// </summary>
+[JsonConverter(typeof(JsonStringEnumConverter<PlayBucket>))]
+public enum PlayBucket
+{
+    /// <summary>Reached &lt; 30% of duration — a skip or a brief partial listen (still past the floor).</summary>
+    Partial,
+
+    /// <summary>Reached 30%–80% of duration — a real listen that was neither a skip nor a finish.</summary>
+    Sampled,
+
+    /// <summary>Reached &gt; 80% of duration — effectively a finished listen.</summary>
+    Complete
+}

DeepDrftModels/Enums/ShareChannel.cs

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+using System.Text.Json.Serialization;
+
+namespace DeepDrftModels.Enums;
+
+/// <summary>
+/// The channel a share was performed through (Phase 16 §1b). Today both originate from
+/// <c>SharePopover</c>'s clipboard actions; a future native/Web-Share button would add a channel
+/// without reshaping the metric. Serialized as its string name on the wire.
+/// </summary>
+[JsonConverter(typeof(JsonStringEnumConverter<ShareChannel>))]
+public enum ShareChannel
+{
+    /// <summary>Copy-link — the canonical track or release URL placed on the clipboard.</summary>
+    Link,
+
+    /// <summary>Copy-embed — the <c>&lt;iframe&gt;</c> snippet for the single-track FramePlayer.</summary>
+    Embed
+}
+
+/// <summary>
+/// What a share targets (Phase 16 §1b). Tracks and releases are both shareable; the popover knows
+/// which it is at the point of the action, so no server-side resolution is needed for shares.
+/// Serialized as its string name on the wire.
+/// </summary>
+[JsonConverter(typeof(JsonStringEnumConverter<ShareTargetType>))]
+public enum ShareTargetType
+{
+    /// <summary>The share targets a single track, addressed by its vault <c>EntryKey</c>.</summary>
+    Track,
+
+    /// <summary>The share targets a release, addressed by its public <c>EntryKey</c>.</summary>
+    Release
+}