feat(rendering): add shader-vector fullscreen SRP seam

Generalize the native fullscreen pass and descriptor plumbing so managed SRP can request shader-driven fullscreen stages without being locked to the ColorScale path.

Keep ColorScale as a convenience descriptor mapped to the builtin color-scale shader, and add native fullscreen factory coverage for the new shader-vector path.

managed/XCEngine.ScriptCore/ScriptableRenderContext.cs

@@ -54,11 +54,15 @@ namespace XCEngine
                     nameof(pass));
             }
 
+            string shaderPath = pass.shaderPath ?? string.Empty;
+            string passName = pass.passName ?? string.Empty;
             Vector4 vectorPayload = pass.vectorPayload;
             return InternalCalls
                 .Rendering_ScriptableRenderContext_RecordFullscreenPass(
                     m_nativeHandle,
                     (int)pass.type,
+                    shaderPath,
+                    passName,
                     ref vectorPayload);
         }
     }