refactor(srp): unify managed pipeline instance ownership

Move shared managed ScriptableRenderPipeline instance ownership into MonoManagedRenderPipelineAssetRuntime.

Make stage recorders borrow the runtime-owned pipeline instead of creating and destroying private handles.

Add a regression test that locks one CreatePipeline call across multiple stage recorders.

managed/GameScripts/RenderPipelineApiProbe.cs

@@ -1306,6 +1306,23 @@ namespace Gameplay
         }
     }
 
+    public sealed class ManagedRenderPipelineCreateCountObservationProbe
+        : MonoBehaviour
+    {
+        public int ObservedCreatePipelineCallCount;
+
+        public void Start()
+        {
+            ManagedRenderPipelineProbeAsset.CreatePipelineCallCount = 0;
+        }
+
+        public void Update()
+        {
+            ObservedCreatePipelineCallCount =
+                ManagedRenderPipelineProbeAsset.CreatePipelineCallCount;
+        }
+    }
+
     public sealed class ScriptCoreUniversalRenderPipelineSelectionProbe
         : MonoBehaviour
     {