fix(audience-sdk): guard IsExternalInit shim against Unity 6 corlib (SDK-256)

Unity 2021.3 needs an internal IsExternalInit shim so the C# compiler can build records with
init-only properties. Unity 2022.2+ ships a compiler and corlib that include the real type, raising
CS0436 when our shim collides — and on Unity 6's IL2CPP, the linker may bind the wrong definition at
the 'with' expression sites in SetConsent/Reset, throwing MissingMethodException at runtime.

Wrap the shim in #if !UNITY_2022_2_OR_NEWER so only the toolchains that need it declare the type.

Author: ImmutableJeffrey

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

@@ -1,12 +1,20 @@
 #nullable enable
 
+// Unity's bundled compiler in 2022.2+ already includes
+// System.Runtime.CompilerServices.IsExternalInit. Defining our own would
+// raise CS0436 (type conflicts with imported type) and IL2CPP under
+// Unity 6 may link the wrong definition — manifesting as
+// MissingMethodException at the `with` expression sites in SetConsent
+// and Reset. Only declare the shim on the older toolchains that need it.
+#if !UNITY_2022_2_OR_NEWER
 namespace System.Runtime.CompilerServices
 {
     // Unity's .NET runtime doesn't include this type, but the C# compiler
     // needs it to exist to build the ConsentState record below. Declaring
     // an empty one here gives the compiler what it looks for.
     internal static class IsExternalInit { }
 }
+#endif
 
 namespace Immutable.Audience
 {