chore(audience-sdk): nullable annotations on Json + Log; range-check ConsentStore (SDK-256)

- #nullable enable on Json.cs and Log.cs surfaces the real nullability concerns: HashSet<object>?
  for the Json visited-set first call from Serialize, and Action<string>? for Log.Writer (null until
  AudienceUnityHooks.Install runs).
- Replaces reflective Enum.IsDefined in ConsentStore.Load with an explicit range check (raw >=
  ConsentLevel.None && raw <= ConsentLevel.Full); IL2CPP-safe under aggressive stripping where enum
  metadata may be dropped even with link.xml preserve="all".

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/Core/ConsentStore.cs

@@ -36,7 +36,11 @@ internal static void Save(string persistentDataPath, ConsentLevel level)
                 if (!File.Exists(filePath)) return null;
 
                 var text = File.ReadAllText(filePath).Trim();
-                if (int.TryParse(text, out var raw) && Enum.IsDefined(typeof(ConsentLevel), raw))
+                // Range check is reflection-free (Enum.IsDefined uses reflection; under
+                // Unity 6's tighter IL2CPP stripping the metadata it walks may be
+                // stripped even with link.xml preserve="all" on the assembly).
+                if (int.TryParse(text, out var raw)
+                    && raw >= (int)ConsentLevel.None && raw <= (int)ConsentLevel.Full)
                     return (ConsentLevel)raw;
             }
             catch (IOException)

src/Packages/Audience/Runtime/Utility/Json.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.Collections;
 using System.Collections.Generic;
@@ -31,7 +33,7 @@ internal static string Serialize(Dictionary<string, object> data, int indent)
             return sb.ToString();
         }
 
-        private static void WriteValue(StringBuilder sb, object value, int indent, int depth, HashSet<object> visited)
+        private static void WriteValue(StringBuilder sb, object? value, int indent, int depth, HashSet<object>? visited)
         {
             if (value == null)
             {
@@ -81,11 +83,11 @@ private static void WriteValue(StringBuilder sb, object value, int indent, int d
             }
             else
             {
-                WriteString(sb, value.ToString());
+                WriteString(sb, value.ToString() ?? string.Empty);
             }
         }
 
-        private static void WriteObject(StringBuilder sb, Dictionary<string, object> dict, int indent, int depth, HashSet<object> visited)
+        private static void WriteObject(StringBuilder sb, Dictionary<string, object> dict, int indent, int depth, HashSet<object>? visited)
         {
             GuardDepth(depth);
             visited = EnterContainer(dict, visited);
@@ -109,7 +111,7 @@ private static void WriteObject(StringBuilder sb, Dictionary<string, object> dic
             visited.Remove(dict);
         }
 
-        private static void WriteArray(StringBuilder sb, IList list, int indent, int depth, HashSet<object> visited)
+        private static void WriteArray(StringBuilder sb, IList list, int indent, int depth, HashSet<object>? visited)
         {
             GuardDepth(depth);
             visited = EnterContainer(list, visited);
@@ -143,7 +145,7 @@ private static void GuardDepth(int depth)
                     "Check for a cyclic or excessively deep dictionary/list.");
         }
 
-        private static HashSet<object> EnterContainer(object container, HashSet<object> visited)
+        private static HashSet<object> EnterContainer(object container, HashSet<object>? visited)
         {
             visited ??= new HashSet<object>(ReferenceEqualityComparer.Instance);
             if (!visited.Add(container))

src/Packages/Audience/Runtime/Utility/Log.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 
 namespace Immutable.Audience
@@ -9,7 +11,7 @@ internal static class Log
         internal static bool Enabled { get; set; }
 
         // Tests set this to capture output; AudienceUnityHooks sets it to Debug.Log.
-        internal static Action<string> Writer { get; set; }
+        internal static Action<string>? Writer { get; set; }
 
         internal static void Debug(string message)
         {