RHI: Add explicit RenderPass abstraction (BeginRenderPass/EndRenderPass)

New abstractions:
- RHIFramebuffer: Framebuffer interface with Initialize/Bind/GetHandle
- RHIRenderPass: RenderPass interface with AttachmentDesc for load/store actions
- D3D12Framebuffer/D3D12RenderPass: D3D12 implementation
- OpenGLFramebuffer/OpenGLRenderPass: OpenGL implementation (adapted from existing)

RHICommandList changes:
- Added BeginRenderPass(RHIRenderPass*, RHIFramebuffer*, Rect, clearValues...)
- Added EndRenderPass()

Implementation notes:
- D3D12: Uses OMSetRenderTargets + ClearRenderTargetView (fallback from native RenderPass API)
- OpenGL: Uses glBindFramebuffer + glClearBufferfv for LoadOp handling
- Old SetRenderTargets/ClearRenderTarget retained for backward compatibility

All 845 tests pass.

engine/src/RHI/OpenGL/OpenGLCommandList.cpp

@@ -2,6 +2,8 @@
 #include "XCEngine/RHI/OpenGL/OpenGLResourceView.h"
 #include "XCEngine/RHI/OpenGL/OpenGLPipelineState.h"
 #include "XCEngine/RHI/OpenGL/OpenGLShader.h"
+#include "XCEngine/RHI/OpenGL/OpenGLFramebuffer.h"
+#include "XCEngine/RHI/OpenGL/OpenGLRenderPass.h"
 #include <glad/glad.h>
 
 namespace XCEngine {
@@ -508,6 +510,45 @@ void OpenGLCommandList::TransitionBarrier(RHIResourceView* resource, ResourceSta
     glMemoryBarrier(GL_ALL_BARRIER_BITS);
 }
 
+void OpenGLCommandList::BeginRenderPass(RHIRenderPass* renderPass, RHIFramebuffer* framebuffer,
+                                     const Rect& renderArea, uint32_t clearValueCount, const ClearValue* clearValues) {
+    (void)renderArea;
+    (void)clearValueCount;
+    (void)clearValues;
+    
+    if (!framebuffer) return;
+    
+    OpenGLFramebuffer* glFramebuffer = static_cast<OpenGLFramebuffer*>(framebuffer);
+    glFramebuffer->Bind();
+    
+    OpenGLRenderPass* glRenderPass = static_cast<OpenGLRenderPass*>(renderPass);
+    if (glRenderPass) {
+        const AttachmentDesc* colorAttachments = glRenderPass->GetColorAttachments();
+        uint32_t colorCount = glRenderPass->GetColorAttachmentCount();
+        
+        for (uint32_t i = 0; i < colorCount && i < clearValueCount; ++i) {
+            if (colorAttachments[i].loadOp == LoadAction::Clear) {
+                glClearBufferfv(GL_COLOR, i, &clearValues[i].color.r);
+            }
+        }
+        
+        const AttachmentDesc* dsAttachment = glRenderPass->GetDepthStencilAttachment();
+        if (dsAttachment && clearValueCount > colorCount) {
+            if (dsAttachment->loadOp == LoadAction::Clear) {
+                glClearBufferfv(GL_DEPTH, 0, &clearValues[colorCount].depth);
+            }
+            if (dsAttachment->stencilLoadOp == LoadAction::Clear) {
+                float stencilValue = static_cast<float>(clearValues[colorCount].stencil);
+                glClearBufferfv(GL_STENCIL, 0, &stencilValue);
+            }
+        }
+    }
+}
+
+void OpenGLCommandList::EndRenderPass() {
+    glBindFramebuffer(GL_FRAMEBUFFER, 0);
+}
+
 void OpenGLCommandList::SetViewport(const Viewport& viewport) {
     glViewport(static_cast<int>(viewport.topLeftX), static_cast<int>(viewport.topLeftY),
                static_cast<int>(viewport.width), static_cast<int>(viewport.height));