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/flutter_window.cpp

@@ -0,0 +1,71 @@
+#include "flutter_window.h"
+
+#include <optional>
+
+#include "flutter/generated_plugin_registrant.h"
+
+FlutterWindow::FlutterWindow(const flutter::DartProject& project)
+    : project_(project) {}
+
+FlutterWindow::~FlutterWindow() {}
+
+bool FlutterWindow::OnCreate() {
+  if (!Win32Window::OnCreate()) {
+    return false;
+  }
+
+  RECT frame = GetClientArea();
+
+  // The size here must match the window dimensions to avoid unnecessary surface
+  // creation / destruction in the startup path.
+  flutter_controller_ = std::make_unique<flutter::FlutterViewController>(
+      frame.right - frame.left, frame.bottom - frame.top, project_);
+  // Ensure that basic setup of the controller was successful.
+  if (!flutter_controller_->engine() || !flutter_controller_->view()) {
+    return false;
+  }
+  RegisterPlugins(flutter_controller_->engine());
+  SetChildContent(flutter_controller_->view()->GetNativeWindow());
+
+  flutter_controller_->engine()->SetNextFrameCallback([&]() {
+    this->Show();
+  });
+
+  // Flutter can complete the first frame before the "show window" callback is
+  // registered. The following call ensures a frame is pending to ensure the
+  // window is shown. It is a no-op if the first frame hasn't completed yet.
+  flutter_controller_->ForceRedraw();
+
+  return true;
+}
+
+void FlutterWindow::OnDestroy() {
+  if (flutter_controller_) {
+    flutter_controller_ = nullptr;
+  }
+
+  Win32Window::OnDestroy();
+}
+
+LRESULT
+FlutterWindow::MessageHandler(HWND hwnd, UINT const message,
+                              WPARAM const wparam,
+                              LPARAM const lparam) noexcept {
+  // Give Flutter, including plugins, an opportunity to handle window messages.
+  if (flutter_controller_) {
+    std::optional<LRESULT> result =
+        flutter_controller_->HandleTopLevelWindowProc(hwnd, message, wparam,
+                                                      lparam);
+    if (result) {
+      return *result;
+    }
+  }
+
+  switch (message) {
+    case WM_FONTCHANGE:
+      flutter_controller_->engine()->ReloadSystemFonts();
+      break;
+  }
+
+  return Win32Window::MessageHandler(hwnd, message, wparam, lparam);
+}