D3D12: Fix texture ownership semantics and remove GetSwapChain() exposure

This commit fixes the D3D12 texture architecture issues:

1. D3D12Texture ownership semantics:
   - Add m_ownsResource member to track resource ownership
   - InitializeFromExisting() now takes ownsResource parameter (default false)
   - Shutdown() only releases resource if ownsResource is true
   - Initialize() sets m_ownsResource = true for created resources

2. D3D12SwapChain changes:
   - Remove GetSwapChain() method (was exposing native D3D12 API)
   - Change GetBackBuffer() to return reference instead of pointer
   - Back buffers initialized with ownsResource = false

3. minimal/main.cpp updates:
   - Remove gColorRTs[2] array (was duplicating back buffer wrapping)
   - Direct use of gSwapChain.GetBackBuffer(i) instead
   - All references updated to use encapsulated API

4. Documentation:
   - Update TEST_SPEC.md v1.3
   - Remove known limitation 7.2 (minimal GetBuffer issue fixed)
   - Add D3D12_Texture_Architecture_Fix_Plan.md design document

tests/RHI/D3D12/TEST_SPEC.md

@@ -290,22 +290,6 @@ engine/
 
 **状态**: 待修复
 
-### 7.2 minimal GetBuffer 原生调用
-
-**问题**: `minimal/main.cpp` 第 127-129 行仍使用原生 D3D12 API:
-
-```cpp
-ID3D12Resource* buffer = nullptr;
-gSwapChain.GetSwapChain()->GetBuffer(i, IID_PPV_ARGS(&buffer));
-gColorRTs[i].InitializeFromExisting(buffer);
-```
-
-**原因**: SwapChain back buffer 获取后需要通过 `InitializeFromExisting()` 绑定到已有的 `D3D12Texture` 对象。当前没有优雅的封装方案保留此语义。
-
-**影响**: minimal 集成测试仍包含少量原生 D3D12 代码
-
-**状态**: 标记为已知限制，暂不修复
-
 ---
 
 ## 8. 规范更新记录
@@ -315,9 +299,10 @@ gColorRTs[i].InitializeFromExisting(buffer);
 | 1.0 | 2026-03-20 | 初始版本 |
 | 1.1 | 2026-03-20 | 添加 CI 集成章节，补充 Phase 5 内容 |
 | 1.2 | 2026-03-20 | 重构集成测试目录结构，每个测试独立子文件夹，stb 库移至 engine/third_party/stb/ |
+| 1.3 | 2026-03-20 | 修复 minimal GetBuffer 原生调用问题：添加 D3D12Texture 所有权语义，删除 GetSwapChain() 暴露方法，移除 gColorRTs 数组 |
 
 ---
 
-**规范版本**: 1.2  
+**规范版本**: 1.3  
 **最后更新**: 2026-03-20  
 **前置文档**: [tests/TEST_SPEC.md](../TEST_SPEC.md)

tests/RHI/D3D12/integration/minimal/main.cpp

@@ -44,7 +44,6 @@ D3D12CommandAllocator gCommandAllocator;
 D3D12CommandList gCommandList;
 
 // Render targets
-D3D12Texture gColorRTs[2];
 D3D12Texture gDepthStencil;
 D3D12DescriptorHeap gRTVHeap;
 D3D12DescriptorHeap gDSVHeap;
@@ -124,13 +123,11 @@ bool InitD3D12() {
 
     // Create RTVs for back buffers using encapsulated interface
     for (int i = 0; i < 2; i++) {
-        ID3D12Resource* buffer = nullptr;
-        gSwapChain.GetSwapChain()->GetBuffer(i, IID_PPV_ARGS(&buffer));
-        gColorRTs[i].InitializeFromExisting(buffer);
+        D3D12Texture& backBuffer = gSwapChain.GetBackBuffer(i);
 
         CPUDescriptorHandle rtvCpuHandle = gRTVHeap.GetCPUDescriptorHandle(i);
         D3D12_CPU_DESCRIPTOR_HANDLE rtvHandle = { rtvCpuHandle.ptr };
-        gRTVs[i].InitializeAt(device, gColorRTs[i].GetResource(), rtvHandle, nullptr);
+        gRTVs[i].InitializeAt(device, backBuffer.GetResource(), rtvHandle, nullptr);
     }
 
     // Create DSV
@@ -164,7 +161,8 @@ void BeginRender() {
     gCurrentRTIndex = gSwapChain.GetCurrentBackBufferIndex();
 
     // Transition render target
-    gCommandList.TransitionBarrier(gColorRTs[gCurrentRTIndex].GetResource(),
+    D3D12Texture& currentBackBuffer = gSwapChain.GetBackBuffer(gCurrentRTIndex);
+    gCommandList.TransitionBarrier(currentBackBuffer.GetResource(),
         ResourceStates::Present, ResourceStates::RenderTarget);
 
     // Set render targets using encapsulated interface
@@ -189,7 +187,8 @@ void BeginRender() {
 
 // End rendering
 void EndRender() {
-    gCommandList.TransitionBarrier(gColorRTs[gCurrentRTIndex].GetResource(),
+    D3D12Texture& currentBackBuffer = gSwapChain.GetBackBuffer(gCurrentRTIndex);
+    gCommandList.TransitionBarrier(currentBackBuffer.GetResource(),
         ResourceStates::RenderTarget, ResourceStates::Present);
 }
 
@@ -275,7 +274,7 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
                 bool screenshotResult = D3D12Screenshot::Capture(
                     gDevice,
                     gCommandQueue,
-                    gColorRTs[gCurrentRTIndex],
+                    gSwapChain.GetBackBuffer(gCurrentRTIndex),
                     "minimal.ppm"
                 );
                 if (screenshotResult) {