feat(engine): refactor resource mgmt & render pipeline

Refactors engine infrastructure for improved resource/service
management and render pipeline extensibility. Replaces World’s
resource API with a service-based API. Splits IGraphicsEngine’s
RenderFrame into BeginFrame/EndFrame. Adds support for pluggable
render pipelines in RenderSystem. Replaces disposed checks with
Debug.Assert in performance paths. Updates RenderExtractionSystem
and render loop for new APIs. Improves diagnostics and code clarity.

BREAKING CHANGE: Resource API replaced with service API; render
pipeline and frame lifecycle interfaces changed.

src/Runtime/Ghost.Core/Result.cs

@@ -390,6 +390,16 @@ public static class ResultExtensions
         return result;
     }
 
+    public static Result Then(this Result result, Func<Result> func)
+    {
+        if (result.IsFailure)
+        {
+            return Result.Failure(result.Message);
+        }
+
+        return func();
+    }
+
     public static Result<U> Then<T, U>(this Result<T> result, Func<T, Result<U>> func)
     {
         if (result.IsFailure)