性能优化：P0/P1 热路径优化及 DepthBuffer 定点化

- 新增 USE_RGB565_BACKBUFFER 选项，FrameBuffer 内部以 RGB565 存储，present 阶段直接 memcpy，消除逐像素格式转换
- 新增 set_pixel_unsafe / get_depth_unsafe / set_depth_unsafe 快路径，热路径去除重复边界检查
- fill_rect 改为行级批量填充，draw_text/draw_sprite_ex 改用 unsafe 写入
- DepthBuffer 从 float 迁移至 uint16_t 定点深度，clear 支持 memset 快速路径
- TriangleRasterizer 深度测试适配 uint16_t（临时桥接，待全面定点化）
- 修复 USE_RGB565_BACKBUFFER 宏未传递到编译器的问题

CMakeLists.txt

@@ -9,6 +9,7 @@ if(NOT CMAKE_CONFIGURATION_TYPES AND NOT CMAKE_BUILD_TYPE)
 endif()
 
 option(USE_FRAMEBUFFER "Use Linux framebuffer instead of SDL2" OFF)
+option(USE_RGB565_BACKBUFFER "Use RGB565 internal backbuffer to eliminate present conversion on RGB565 fb0" OFF)
 
 set(CORE_SOURCES
     src/Core/Asset/ObjLoader.cpp
@@ -58,6 +59,13 @@ target_include_directories(imx6u_core PUBLIC ${CORE_INCLUDE_DIRS})
 
 if(USE_FRAMEBUFFER)
     target_compile_definitions(imx6u_core PUBLIC USE_FRAMEBUFFER)
+endif()
+
+if(USE_RGB565_BACKBUFFER)
+    target_compile_definitions(imx6u_core PUBLIC USE_RGB565_BACKBUFFER)
+endif()
+
+if(USE_FRAMEBUFFER)
 else()
     if(WIN32)
         if(CMAKE_SIZEOF_VOID_P EQUAL 8)
@@ -106,6 +114,15 @@ function(imx6u_configure_app_target target_name)
             "${SDL2_DLL}"
             "$<TARGET_FILE_DIR:${target_name}>"
         )
+    elseif(SDL2_image_FOUND)
+        add_executable(SpriteAssetTool ${SPRITE_ASSET_TOOL_SOURCES})
+        target_include_directories(SpriteAssetTool PRIVATE
+            src/Core/Asset
+            src/Core/RenderData
+        )
+        target_link_libraries(SpriteAssetTool PRIVATE SDL2::SDL2 SDL2_image::SDL2_image)
+    else()
+        message(STATUS "SpriteAssetTool disabled: SDL2_image was not found")
     endif()
 
     if(MSVC)