refactor(audience): enable nullable reference types on Slice 4 files

- Runtime/Unity/AudienceUnityHooks.cs: directive only (all references
  come from UnityEngine which is typed conservatively).
- ImmutableAudience.cs: LaunchContextProvider field (added by the
  auto-context commit on top of the already-annotated file) → nullable;
  unityContext local in FireGameLaunch → nullable (receives provider
  result that may be null on exception).

Compile-time annotations only; zero runtime behaviour change.

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/ImmutableAudience.cs

@@ -32,7 +32,7 @@ public static class ImmutableAudience
 
         // AudienceUnityHooks sets this so game_launch can auto-include
         // Unity context without the core referencing UnityEngine.
-        internal static Func<Dictionary<string, object>> LaunchContextProvider;
+        internal static Func<Dictionary<string, object>>? LaunchContextProvider;
 
         // Starts the SDK. Call once at launch.
         public static void Init(AudienceConfig config)
@@ -569,7 +569,7 @@ private static void FireGameLaunch(AudienceConfig config, ConsentLevel consentAt
             var provider = LaunchContextProvider;
             if (provider != null)
             {
-                Dictionary<string, object> unityContext = null;
+                Dictionary<string, object>? unityContext = null;
                 try { unityContext = provider(); }
                 catch (Exception ex)
                 {

src/Packages/Audience/Runtime/Unity/AudienceUnityHooks.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System.Collections.Generic;
 using UnityEngine;