refactor(audience): enable nullable reference types on Slice 3 files

- AudienceConfig.cs: PublishableKey, DistributionPlatform,
  OnError, PersistentDataPath, HttpHandler → nullable. PackageVersion
  stays non-null (has default). Keep PublishableKey nullable at the
  property since Init throws the actual guard.
- ConsentLevel.cs: directive only (no annotations needed).
- Transport/EventQueue.cs: Enqueue / EnqueueChecked msg param → nullable
  (existing guard `if (_disposed || msg == null) return`); stillAllowed
  param on EnqueueChecked → Func<bool>?.
- Transport/DiskStore.cs: TryReadMessage out param →
  [NotNullWhen(true)] out Dictionary<string, object>? msg so callers
  flow-narrow to non-null on the true branch.
- ImmutableAudience.cs: static reference fields (_config, _store,
  _queue, _transport, _controlClient, _sendTimer, _userId,
  DefaultPersistentDataPathProvider) → nullable. Public API default
  params (traits, properties, userId for DeleteData) → nullable.
  Identify/Alias string-overload identityType / fromType / toType
  → nullable (body already validates and drops on null).
  NotifyErrorCallback onError param → nullable.
  SnapshotCallerDict and Enqueue internal params → nullable.
  One `!` used at ConsentStore.Save call because Init's guard is
  out of the compiler's flow-analysis reach.

Compile-time annotations only; zero runtime behaviour change.

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/AudienceConfig.cs

@@ -1,18 +1,20 @@
+#nullable enable
+
 using System;
 
 namespace Immutable.Audience
 {
     // Configuration passed to ImmutableAudience.Init.
     public class AudienceConfig
     {
-        // Studio API key.
-        public string PublishableKey { get; set; }
+        // Studio API key. Required — Init throws if null.
+        public string? PublishableKey { get; set; }
 
         // Initial consent level.
         public ConsentLevel Consent { get; set; } = ConsentLevel.None;
 
         // Distribution platform the game is running on.
-        public string DistributionPlatform { get; set; }
+        public string? DistributionPlatform { get; set; }
 
         // Enable debug logging.
         public bool Debug { get; set; } = false;
@@ -24,10 +26,11 @@ public class AudienceConfig
         public int FlushSize { get; set; } = Constants.DefaultFlushSize;
 
         // Optional error callback.
-        public Action<AudienceError> OnError { get; set; }
+        public Action<AudienceError>? OnError { get; set; }
 
         // Directory the SDK uses for identity, consent, and queued events.
-        public string PersistentDataPath { get; set; }
+        // Unity hooks populate this from Application.persistentDataPath.
+        public string? PersistentDataPath { get; set; }
 
         // Library version sent on every message.
         public string PackageVersion { get; set; } = Constants.LibraryVersion;
@@ -36,6 +39,6 @@ public class AudienceConfig
         public int ShutdownFlushTimeoutMs { get; set; } = 2_000;
 
         // Test seam for HttpTransport; not part of the public API.
-        internal System.Net.Http.HttpMessageHandler HttpHandler { get; set; }
+        internal System.Net.Http.HttpMessageHandler? HttpHandler { get; set; }
     }
 }

src/Packages/Audience/Runtime/ConsentLevel.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Immutable.Audience
 {
     // How much data the Audience SDK is allowed to collect.

src/Packages/Audience/Runtime/ImmutableAudience.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Net.Http;
@@ -11,22 +13,22 @@ public static class ImmutableAudience
     {
         // Reference fields are written inside _initLock; readers fence off the volatile _initialized load.
         // _consent and _userId are mutated outside the lock and need volatile themselves.
-        private static AudienceConfig _config;
-        private static DiskStore _store;
-        private static EventQueue _queue;
-        private static HttpTransport _transport;
-        private static HttpClient _controlClient;
-        private static Timer _sendTimer;
+        private static AudienceConfig? _config;
+        private static DiskStore? _store;
+        private static EventQueue? _queue;
+        private static HttpTransport? _transport;
+        private static HttpClient? _controlClient;
+        private static Timer? _sendTimer;
         private static volatile ConsentLevel _consent;
-        private static volatile string _userId;
+        private static volatile string? _userId;
         private static volatile bool _initialized;
         private static readonly object _initLock = new object();
 
         // AudienceUnityHooks sets this at SubsystemRegistration so Unity studios
         // can omit PersistentDataPath from AudienceConfig and Init will fill it
         // from Application.persistentDataPath. Non-Unity callers must still set
         // PersistentDataPath on the config.
-        internal static Func<string> DefaultPersistentDataPathProvider;
+        internal static Func<string>? DefaultPersistentDataPathProvider;
 
         // Starts the SDK. Call once at launch.
         public static void Init(AudienceConfig config)
@@ -119,7 +121,7 @@ public static void Track(IEvent evt)
         }
 
         // Send a custom event.
-        public static void Track(string eventName, Dictionary<string, object> properties = null)
+        public static void Track(string eventName, Dictionary<string, object>? properties = null)
         {
             if (!CanTrack()) return;
             if (string.IsNullOrEmpty(eventName))
@@ -142,12 +144,12 @@ public static void Track(string eventName, Dictionary<string, object> properties
         // -----------------------------------------------------------------
 
         // Attach a known user id to subsequent events.
-        public static void Identify(string userId, IdentityType identityType, Dictionary<string, object> traits = null) =>
+        public static void Identify(string userId, IdentityType identityType, Dictionary<string, object>? traits = null) =>
             Identify(userId, identityType.ToLowercaseString(), traits);
 
         // Attach a known user id to subsequent events. String overload for
         // providers not in IdentityType.
-        public static void Identify(string userId, string identityType, Dictionary<string, object> traits = null)
+        public static void Identify(string userId, string? identityType, Dictionary<string, object>? traits = null)
         {
             if (!_initialized) return;
 
@@ -185,7 +187,7 @@ public static void Alias(string fromId, IdentityType fromType, string toId, Iden
 
         // Link two user ids for the same player. String overload for
         // providers not in IdentityType.
-        public static void Alias(string fromId, string fromType, string toId, string toType)
+        public static void Alias(string fromId, string? fromType, string toId, string? toType)
         {
             if (!_initialized) return;
 
@@ -225,7 +227,7 @@ public static void Reset()
         }
 
         // Ask the backend to erase this player's data.
-        public static void DeleteData(string userId = null)
+        public static void DeleteData(string? userId = null)
         {
             if (!_initialized) return;
 
@@ -273,7 +275,7 @@ public static void DeleteData(string userId = null)
             });
         }
 
-        private static void NotifyErrorCallback(Action<AudienceError> onError, AudienceErrorCode code, string message)
+        private static void NotifyErrorCallback(Action<AudienceError>? onError, AudienceErrorCode code, string message)
         {
             if (onError == null) return;
             try
@@ -312,7 +314,8 @@ public static void SetConsent(ConsentLevel level)
 
             try
             {
-                ConsentStore.Save(config.PersistentDataPath, level);
+                // PersistentDataPath is validated non-null in Init; compiler can't propagate that.
+                ConsentStore.Save(config.PersistentDataPath!, level);
             }
             catch (Exception ex)
             {
@@ -486,10 +489,10 @@ private static bool CanTrack()
         }
 
         // Shallow-copy the caller's dict so a post-call mutation cannot race the drain-thread serialiser.
-        private static Dictionary<string, object> SnapshotCallerDict(Dictionary<string, object> src) =>
+        private static Dictionary<string, object>? SnapshotCallerDict(Dictionary<string, object>? src) =>
             src != null ? new Dictionary<string, object>(src) : null;
 
-        private static void Enqueue(Dictionary<string, object> msg)
+        private static void Enqueue(Dictionary<string, object>? msg)
         {
             var queue = _queue;
             if (queue == null) return;

src/Packages/Audience/Runtime/Transport/DiskStore.cs

@@ -1,5 +1,8 @@
+#nullable enable
+
 using System;
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 
@@ -140,7 +143,7 @@ private void ApplyAnonymousDowngradeToFile(string path)
         private static bool IsIdentityMessage(string type) =>
             type == MessageTypes.Identify || type == MessageTypes.Alias;
 
-        private static bool TryReadMessage(string path, out Dictionary<string, object> msg)
+        private static bool TryReadMessage(string path, [NotNullWhen(true)] out Dictionary<string, object>? msg)
         {
             msg = null;
             string json;

src/Packages/Audience/Runtime/Transport/EventQueue.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
@@ -51,7 +53,7 @@ internal EventQueue(DiskStore store, int flushIntervalSeconds, int flushSize)
         // The dictionary is not copied -- callers must not mutate it after
         // enqueue. Serialisation happens on the drain thread so Track() stays
         // allocation-light.
-        internal void Enqueue(Dictionary<string, object> msg)
+        internal void Enqueue(Dictionary<string, object>? msg)
         {
             if (_disposed || msg == null) return;
 
@@ -65,7 +67,7 @@ internal void Enqueue(Dictionary<string, object> msg)
         // Enqueues under _drainLock, re-checking stillAllowed inside the lock.
         // Closes the window where a concurrent PurgeAll could complete between
         // the caller's check and the enqueue, leaking the event past revocation.
-        internal void EnqueueChecked(Dictionary<string, object> msg, Func<bool> stillAllowed)
+        internal void EnqueueChecked(Dictionary<string, object>? msg, Func<bool>? stillAllowed)
         {
             if (_disposed || msg == null) return;