feat(srp): add renderer-driven pipeline backbone

Introduce renderer-driven and renderer-backed managed pipeline base types in the Universal package.

Move shared renderer-data/default-renderer ownership out of UniversalRenderPipelineAsset, migrate probe assets onto the generic seam, and expose renderer recording/request context types for future SRP expansion.

Update scripting API-surface expectations and validate with build, unit tests, scripting tests, and old editor smoke.

managed/XCEngine.ScriptCore/Rendering/Universal/RendererBackedRenderPipeline.cs

@@ -0,0 +1,24 @@
+using XCEngine;
+
+namespace XCEngine.Rendering.Universal
+{
+    public class RendererBackedRenderPipeline
+        : RendererDrivenRenderPipeline
+    {
+        private readonly RendererBackedRenderPipelineAsset m_asset;
+
+        public RendererBackedRenderPipeline(
+            RendererBackedRenderPipelineAsset asset)
+        {
+            m_asset = asset;
+        }
+
+        protected override ScriptableRenderer ResolveRenderer(
+            RendererRecordingContext context)
+        {
+            return m_asset != null
+                ? m_asset.GetDefaultRenderer()
+                : null;
+        }
+    }
+}