feat(srp): add shader pass selection for scene draws

- let DrawingSettings carry an optional shaderPassName across managed and native scene draw APIs
- allow RenderObjectsRendererFeature to author explicit ForwardLit or Unlit scene draws
- exercise the new scene draw pass selection seam in the project render pipeline probe

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

@@ -3,16 +3,22 @@ namespace XCEngine.Rendering
     public struct DrawingSettings
     {
         public string overrideMaterialPath;
+        public string shaderPassName;
 
         public bool hasOverrideMaterial =>
             !string.IsNullOrEmpty(
                 overrideMaterialPath);
 
+        public bool hasShaderPassName =>
+            !string.IsNullOrEmpty(
+                shaderPassName);
+
         public static DrawingSettings CreateDefault()
         {
             return new DrawingSettings
             {
-                overrideMaterialPath = string.Empty
+                overrideMaterialPath = string.Empty,
+                shaderPassName = string.Empty
             };
         }
     }