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/GameScripts/RenderPipelineApiProbe.cs

@@ -2,6 +2,12 @@ using XCEngine;
 
 namespace Gameplay
 {
+    internal static class BuiltinShaderPaths
+    {
+        public const string ColorScalePostProcess =
+            "builtin://shaders/color-scale-post-process";
+    }
+
     public sealed class LegacyRenderPipelineApiProbeAsset : RenderPipelineAsset
     {
     }
@@ -142,11 +148,15 @@ namespace Gameplay
             return context != null &&
                    context.stage == CameraFrameStage.PostProcess &&
                    context.RecordFullscreenPass(
-                       FullscreenPassDescriptor.CreateColorScale(
-                           new Vector4(1.10f, 0.95f, 0.90f, 1.0f))) &&
+                       FullscreenPassDescriptor.CreateShaderVector(
+                           BuiltinShaderPaths.ColorScalePostProcess,
+                           new Vector4(1.10f, 0.95f, 0.90f, 1.0f),
+                           "ColorScale")) &&
                    context.RecordFullscreenPass(
-                       FullscreenPassDescriptor.CreateColorScale(
-                           new Vector4(0.95f, 1.05f, 1.10f, 1.0f)));
+                       FullscreenPassDescriptor.CreateShaderVector(
+                           BuiltinShaderPaths.ColorScalePostProcess,
+                           new Vector4(0.95f, 1.05f, 1.10f, 1.0f),
+                           "ColorScale"));
         }
     }