fix(audience): init does not mutate caller's AudienceConfig

Init auto-fills PersistentDataPath from DefaultPersistentDataPathProvider.
The old code wrote that back onto the caller's config object — a
surprising side-effect for callers who reuse the config.

Clone via MemberwiseClone before the fill-in. Reference-typed properties
(OnError, HttpHandler) are intentionally shared — cloning delegates
and handlers would break them.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/AudienceConfig.cs

@@ -40,5 +40,10 @@ public class AudienceConfig
 
         // Test seam for HttpTransport; not part of the public API.
         internal System.Net.Http.HttpMessageHandler? HttpHandler { get; set; }
+
+        // Shallow copy so Init can populate PersistentDataPath without mutating
+        // the caller's object. Reference-typed properties (OnError, HttpHandler)
+        // are intentionally shared — cloning delegates/handlers is wrong.
+        internal AudienceConfig Clone() => (AudienceConfig)MemberwiseClone();
     }
 }

src/Packages/Audience/Runtime/ImmutableAudience.cs

@@ -50,6 +50,8 @@ public static void Init(AudienceConfig config)
             if (string.IsNullOrEmpty(config.PublishableKey))
                 throw new ArgumentException("PublishableKey is required", nameof(config));
 
+            // Clone so Init's PersistentDataPath fill-in does not mutate the caller's object.
+            config = config.Clone();
             if (string.IsNullOrEmpty(config.PersistentDataPath))
                 config.PersistentDataPath = DefaultPersistentDataPathProvider?.Invoke();
             if (string.IsNullOrEmpty(config.PersistentDataPath))