feat(srp): add explicit skybox draw primitive

- add ScriptableRenderContext.DrawSkybox as a public main-scene draw primitive
- route default RecordScene and DrawSkyboxPass recording through explicit draw calls
- expose the new DrawSkybox API in the scriptable render context probe

managed/XCEngine.ScriptCore/Rendering/Core/ScriptableRenderContext.cs

@@ -36,13 +36,13 @@ namespace XCEngine.Rendering
         public bool RecordScene()
         {
             return RecordBeforeOpaqueInjection() &&
-                   RecordOpaqueScenePhase() &&
+                   DrawOpaqueRenderers() &&
                    RecordAfterOpaqueInjection() &&
                    RecordBeforeSkyboxInjection() &&
-                   RecordSkyboxScenePhase() &&
+                   DrawSkybox() &&
                    RecordAfterSkyboxInjection() &&
                    RecordBeforeTransparentInjection() &&
-                   RecordTransparentScenePhase() &&
+                   DrawTransparentRenderers() &&
                    RecordAfterTransparentInjection();
         }
 
@@ -124,6 +124,12 @@ namespace XCEngine.Rendering
                 SceneRenderPhase.Skybox);
         }
 
+        public bool DrawSkybox()
+        {
+            return RecordScenePhase(
+                SceneRenderPhase.Skybox);
+        }
+
         public bool RecordTransparentScenePhase()
         {
             return RecordScenePhase(