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.

tests/scripting/test_mono_script_runtime.cpp

@@ -679,7 +679,7 @@ TEST_F(
 
 TEST_F(
     MonoScriptRuntimeTest,
-    ManagedStageRecorderRecordsPostProcessThroughScriptableRenderContext) {
+    ManagedStageRecorderRecordsShaderVectorPostProcessThroughScriptableRenderContext) {
     const auto bridge =
         XCEngine::Rendering::Pipelines::GetManagedRenderPipelineBridge();
     ASSERT_NE(bridge, nullptr);