- 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
- extract DrawSkyboxPass from UniversalRenderer into a formal package pass primitive
- keep the default Universal skybox stage wired through the new reusable pass
- reduce direct scene phase recording leakage inside the package layer
