Add ManagedObjectReferenceManager for CoreCLR

Add a new ManagedObjectReferenceManager that maintains GREF and WREF
counters entirely in managed code using Interlocked operations, with
zero P/Invoke overhead. Logging is disabled (LogGlobalReferenceMessages
and LogLocalReferenceMessages return false). The gref_gc_threshold
check for triggering full GC is preserved.

AndroidObjectReferenceManager is kept unchanged for Mono.
CoreCLR uses ManagedObjectReferenceManager, selected at runtime in
AndroidRuntimeOptions. NativeAOT uses the existing
Java.Interop.ManagedObjectReferenceManager with null log writers.

Author: simonrozsival

src/Microsoft.Android.Runtime.NativeAOT/Java.Interop/JreRuntime.cs

@@ -62,7 +62,7 @@ static NativeAotRuntimeOptions CreateJreVM (NativeAotRuntimeOptions builder)
 #endif  // NET
 
 			builder.ValueManager            ??= ManagedValueManager.GetOrCreateInstance();
-			builder.ObjectReferenceManager  ??= new Android.Runtime.AndroidObjectReferenceManager ();
+			builder.ObjectReferenceManager  ??= new ManagedObjectReferenceManager (null, null);
 
 			if (builder.InvocationPointer != IntPtr.Zero || builder.EnvironmentPointer != IntPtr.Zero)
 				return builder;

src/Mono.Android/Android.Runtime/AndroidRuntime.cs

@@ -13,6 +13,8 @@
 using Java.Interop.Tools.TypeNameMappings;
 using System.Diagnostics.CodeAnalysis;
 
+using RuntimeFeature = Microsoft.Android.Runtime.RuntimeFeature;
+
 #if JAVA_INTEROP
 namespace Android.Runtime {
 
@@ -94,7 +96,9 @@ public AndroidRuntimeOptions (IntPtr jnienv,
 			EnvironmentPointer      = jnienv;
 			ClassLoader             = new JniObjectReference (classLoader, JniObjectReferenceType.Global);
 			InvocationPointer       = vm;
-			ObjectReferenceManager  = new AndroidObjectReferenceManager ();
+			ObjectReferenceManager  = RuntimeFeature.IsCoreClrRuntime
+				? new ManagedObjectReferenceManager ()
+				: new AndroidObjectReferenceManager ();
 			TypeManager             = typeManager;
 			ValueManager            = valueManager;
 			UseMarshalMemberBuilder = false;
@@ -244,6 +248,84 @@ public override void DeleteWeakGlobalReference (ref JniObjectReference value)
 		}
 	}
 
+	/// <summary>
+	/// Managed object reference manager that maintains GREF and WREF counters entirely in
+	/// managed code using Interlocked operations, avoiding all P/Invoke overhead.
+	/// Logging uses managed TextWriter streams — no RuntimeNativeMethods calls at all.
+	/// Used for CoreCLR; Mono uses <see cref="AndroidObjectReferenceManager"/> instead.
+	/// </summary>
+	internal class ManagedObjectReferenceManager : JniRuntime.JniObjectReferenceManager {
+
+		int _grefc;
+		int _weak_grefc;
+
+		public override int GlobalReferenceCount => Volatile.Read (ref _grefc);
+		public override int WeakGlobalReferenceCount => Volatile.Read (ref _weak_grefc);
+
+		public override bool LogGlobalReferenceMessages => false;
+		public override bool LogLocalReferenceMessages  => false;
+
+		public override void WriteLocalReferenceLine (string format, params object?[] args)
+		{
+		}
+
+		public override void WriteGlobalReferenceLine (string format, params object?[] args)
+		{
+		}
+
+		public override JniObjectReference CreateLocalReference (JniObjectReference value, ref int localReferenceCount)
+		{
+			return base.CreateLocalReference (value, ref localReferenceCount);
+		}
+
+		public override void DeleteLocalReference (ref JniObjectReference value, ref int localReferenceCount)
+		{
+			base.DeleteLocalReference (ref value, ref localReferenceCount);
+		}
+
+		public override void CreatedLocalReference (JniObjectReference value, ref int localReferenceCount)
+		{
+			base.CreatedLocalReference (value, ref localReferenceCount);
+		}
+
+		public override IntPtr ReleaseLocalReference (ref JniObjectReference value, ref int localReferenceCount)
+		{
+			return base.ReleaseLocalReference (ref value, ref localReferenceCount);
+		}
+
+		public override JniObjectReference CreateGlobalReference (JniObjectReference value)
+		{
+			var r = base.CreateGlobalReference (value);
+			int gc = Interlocked.Increment (ref _grefc);
+
+			if (gc >= JNIEnvInit.gref_gc_threshold) {
+				Logger.Log (LogLevel.Warn, "monodroid-gc", gc + " outstanding GREFs. Performing a full GC!");
+				System.GC.Collect ();
+			}
+
+			return r;
+		}
+
+		public override void DeleteGlobalReference (ref JniObjectReference value)
+		{
+			Interlocked.Decrement (ref _grefc);
+			base.DeleteGlobalReference (ref value);
+		}
+
+		public override JniObjectReference CreateWeakGlobalReference (JniObjectReference value)
+		{
+			var r = base.CreateWeakGlobalReference (value);
+			Interlocked.Increment (ref _weak_grefc);
+			return r;
+		}
+
+		public override void DeleteWeakGlobalReference (ref JniObjectReference value)
+		{
+			Interlocked.Decrement (ref _weak_grefc);
+			base.DeleteWeakGlobalReference (ref value);
+		}
+	}
+
 	class AndroidTypeManager : JniRuntime.JniTypeManager {
 		struct JniRemappingReplacementMethod
 		{