Split vulkan_window.cpp by responsibility and render() into phases

VulkanWindow's 3000-line TU becomes sdl_window.cpp (window, events,
cursor, screenshot BMP), vk_device.cpp (device, swapchain, MSAA,
framebuffers), vk_pipelines.cpp (render passes, pipelines),
vk_resources.cpp (buffers, render targets, geometry), vk_textures.cpp
(textures, samplers, descriptors), vk_frame.cpp (render) and
ui_batches.cpp; vulkan_window.cpp keeps construction and timing stats.
Function bodies move verbatim.

render() now reads fence/acquire -> prepare_draws -> prepare_ui ->
write_draw_states -> record_passes -> record_readbacks -> submit ->
finish_readbacks -> present, with FrameDraws / Readback carrying the
locals that used to live across 500 lines.

Verified: login-screen screenshot byte-identical to HEAD; game-world
draw/vertex/index/UI counts identical; ctest, port_gate macos+android,
Android build pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/PORT-PLAN.md

@@ -64,7 +64,11 @@ src/platform/ScriptLib/PythonBoot.cpp        # 解释器启动、RunMainScript
 third_party/cpython-2.7.18/                  # 静态库
 src/host/main.cpp                            # SDL3 宿主入口：命令行、合成基准
 src/host/live_client.cpp                     # run_live_client：40250 客户端主循环
-src/host/vulkan_window.cpp                   # SDL 窗口/事件 + Vulkan 渲染器（回放 Render3DDraw / UIRenderCommand）
+src/host/vulkan_window.{h,cpp}               # VulkanWindow：构造/析构 + 帧计时统计（回放 Render3DDraw / UIRenderCommand）
+src/host/sdl_window.cpp                      # SDL 窗口/事件/光标/屏幕键盘/截图 BMP
+src/host/vk_{device,pipelines,resources,textures}.cpp  # 交换链 / 渲染通道+管线 / 缓冲+RT+几何 / 纹理+采样器+描述符
+src/host/vk_frame.cpp                        # render()：prepare_draws → prepare_ui → write_draw_states → record_passes → readback
+src/host/ui_batches.cpp                      # UIRenderCommand → UiBatch 合批
 src/host/frame_policy.cpp                    # 帧率策略（FrameRatePolicy）
 src/host/native_audio.cpp                    # MilesLib AudioCommand → SDL 音频流
 src/host/audio_decode.cpp                    # .wav（手写 PCM 解析）/.mp3（dr_mp3）→ 44.1 kHz 立体声 float，全平台同一路径