Remove CPU fallback from GPU context pool

PerfectNumbers.Core/Gpu/GpuContextPool.cs

@@ -145,8 +145,7 @@ public static class GpuContextPool
 
         public object ExecutionLock => _ctx.ExecutionLock;
 
-
+        internal void ReturnToPool()
-                public void Dispose()
         {
             if (disposed)
             {
@@ -156,6 +155,10 @@ public static class GpuContextPool
             Return(_ctx);
             disposed = true;
         }
-    }
-}
 
+        public void Dispose()
+        {
+            ReturnToPool();
+        }
+    }
+}

PerfectNumbers.Core/Gpu/GpuKernelPool.cs

@@ -396,7 +396,10 @@ public ref struct GpuKernelLease(IDisposable limiter, GpuContextLease gpu, Kerne
                 _stream?.Dispose();
                 _stream = null;
 
-                _gpu?.Dispose();
+                if (_gpu.HasValue)
+                {
+                    _gpu.Value.ReturnToPool();
+                }
                 _gpu = null;
 
                 _executionLock = null;