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/FullscreenPassDescriptor.cs

@@ -2,12 +2,15 @@ namespace XCEngine
 {
     public enum FullscreenPassType
     {
-        ColorScale = 0
+        ColorScale = 0,
+        ShaderVector = 1
     }
 
     public struct FullscreenPassDescriptor
     {
         public FullscreenPassType type;
+        public string shaderPath;
+        public string passName;
         public Vector4 vectorPayload;
 
         public static FullscreenPassDescriptor CreateColorScale(
@@ -15,16 +18,33 @@ namespace XCEngine
         {
             FullscreenPassDescriptor descriptor = new FullscreenPassDescriptor();
             descriptor.type = FullscreenPassType.ColorScale;
+            descriptor.shaderPath = string.Empty;
+            descriptor.passName = string.Empty;
             descriptor.vectorPayload = colorScale;
             return descriptor;
         }
 
+        public static FullscreenPassDescriptor CreateShaderVector(
+            string shaderPath,
+            Vector4 vectorPayload,
+            string passName = null)
+        {
+            FullscreenPassDescriptor descriptor = new FullscreenPassDescriptor();
+            descriptor.type = FullscreenPassType.ShaderVector;
+            descriptor.shaderPath = shaderPath ?? string.Empty;
+            descriptor.passName = passName ?? string.Empty;
+            descriptor.vectorPayload = vectorPayload;
+            return descriptor;
+        }
+
         public bool IsValid()
         {
             switch (type)
             {
                 case FullscreenPassType.ColorScale:
                     return true;
+                case FullscreenPassType.ShaderVector:
+                    return !string.IsNullOrEmpty(shaderPath);
                 default:
                     return false;
             }

managed/XCEngine.ScriptCore/InternalCalls.cs

@@ -408,6 +408,8 @@ namespace XCEngine
             Rendering_ScriptableRenderContext_RecordFullscreenPass(
                 ulong nativeHandle,
                 int passType,
+                string shaderPath,
+                string passName,
                 ref Vector4 vectorPayload);
 
         [MethodImpl(MethodImplOptions.InternalCall)]

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);
         }
     }