feat: Windows PavoApi ffi 直打后端（Phase 3 代码完成，待实机验收）

- lib/print/pavo/：PavoApiW_x64.dll 的 dart:ffi 绑定（结构体布局对齐 C#，W 后缀导出名回退兼容）
- pavo_printer.dart：枚举（UTF-8/UTF-16 自动解码）/状态/色带信息/单卡直打/忙碌轮询（3 分钟超时）/LayerHolder 非托管内存管理
- pavo_backend.dart：全部 ffi 调用与像素转换跑在 Isolate.run，主 isolate HTTP 服务零阻塞
- lib/image/card_image.dart：PNG data URL → 纵横比选卡面 → 手动 alpha 铺白底 → BGR24/Gray8 紧凑像素（image 包 compositeImage 对全透明像素非 src-over 语义，改为逐像素混合对齐 C#）
- windows/ 目录：从 Flutter SDK windows.tmpl 手动实例化（mac 上 flutter create 拒绝生成的绕过），窗口标题"饭卡打印助手"
- windows/vendor/：收录 SDK 的 PavoApiW_x64.dll/PavoCardSDK_x64.dll/HeatTB3.dll/heatthcd.bin，CMake POST_BUILD 自动拷到 exe 旁
- backend.dart 工厂接通 Windows 分支；analyze 零问题、单测 12 个全过

windows/runner/main.cpp

@@ -0,0 +1,43 @@
+#include <flutter/dart_project.h>
+#include <flutter/flutter_view_controller.h>
+#include <windows.h>
+
+#include "flutter_window.h"
+#include "utils.h"
+
+int APIENTRY wWinMain(_In_ HINSTANCE instance, _In_opt_ HINSTANCE prev,
+                      _In_ wchar_t *command_line, _In_ int show_command) {
+  // Attach to console when present (e.g., 'flutter run') or create a
+  // new console when running with a debugger.
+  if (!::AttachConsole(ATTACH_PARENT_PROCESS) && ::IsDebuggerPresent()) {
+    CreateAndAttachConsole();
+  }
+
+  // Initialize COM, so that it is available for use in the library and/or
+  // plugins.
+  ::CoInitializeEx(nullptr, COINIT_APARTMENTTHREADED);
+
+  flutter::DartProject project(L"data");
+
+  std::vector<std::string> command_line_arguments =
+      GetCommandLineArguments();
+
+  project.set_dart_entrypoint_arguments(std::move(command_line_arguments));
+
+  FlutterWindow window(project);
+  Win32Window::Point origin(10, 10);
+  Win32Window::Size size(1280, 720);
+  if (!window.Create(L"饭卡打印助手", origin, size)) {
+    return EXIT_FAILURE;
+  }
+  window.SetQuitOnClose(true);
+
+  ::MSG msg;
+  while (::GetMessage(&msg, nullptr, 0, 0)) {
+    ::TranslateMessage(&msg);
+    ::DispatchMessage(&msg);
+  }
+
+  ::CoUninitialize();
+  return EXIT_SUCCESS;
+}