fix(cache): correct CacheEntry struct accessibility to internal

Changed CacheEntry structs from private to internal to match the
accessibility of the Cache fields that expose them. Fixes CS0052
compilation errors in NinoSerializer.cs and NinoDeserializer.cs.

Author: claude

src/Nino.Core/NinoDeserializer.cs

@@ -193,13 +193,13 @@ public static class CachedDeserializer<T>
         internal static readonly bool IsSimpleType = !IsReferenceOrContainsReferences && !HasBaseType;
 
         // Inline cache entries - pack type ID and delegate together for better cache locality
-        private struct CacheEntry
+        internal struct CacheEntry
         {
             public int TypeId;
             public DeserializeDelegate<T> Deserializer;
         }
 
-        private struct CacheEntryRef
+        internal struct CacheEntryRef
         {
             public int TypeId;
             public DeserializeDelegateRef<T> Deserializer;

src/Nino.Core/NinoSerializer.cs

@@ -189,7 +189,7 @@ public static class CachedSerializer<T>
         internal static readonly bool IsSimpleType = !IsReferenceOrContainsReferences && !HasBaseType;
 
         // Inline cache entry - packs type handle and delegate together for better cache locality
-        private struct CacheEntry
+        internal struct CacheEntry
         {
             public IntPtr TypeHandle;
             public SerializeDelegate<T> Serializer;