refactor: 新建 Platform 平台差异层，主工程 P/Invoke 全部迁出

- 新增 Platform.Abstractions（接口+NoOp+PlatformLog 日志桥）、
  Platform.Windows、Platform.MacOS、Platform.Android 四个项目
- 迁移电源管理、原生对话框、桌面层嵌入、窗口置底/区域穿透、
  DWM 互操作、图标服务、包标识查询等全部 Windows P/Invoke 实现
- 主工程保留静态工厂门面，调用点不变；DllImport 扫描为零
- 更新 WindowPassthroughServiceTests 指向新位置（20/20 通过）

LanMountainDesktop.Platform.Abstractions/IWindowPassthroughServices.cs

@@ -0,0 +1,68 @@
+using Avalonia;
+using Avalonia.Controls;
+
+namespace LanMountainDesktop.Platform.Abstractions;
+
+/// <summary>
+/// 窗口置底服务接口。使组件窗口保持在桌面层（Z 序最底）。
+/// </summary>
+public interface IWindowBottomMostService
+{
+    void SetupBottomMost(Window window);
+    void SendToBottom(Window window);
+    PixelPoint GetScreenPosition(Window window);
+    bool SetScreenPosition(Window window, PixelPoint position, bool queueOnFailure = false);
+    bool IsBottomMostSupported { get; }
+}
+
+/// <summary>
+/// 窗口交互区域定义。区域外的点击穿透到桌面。
+/// </summary>
+public readonly record struct WindowInteractiveRegion(
+    Rect Bounds,
+    double CornerRadius,
+    Matrix? ClientToRegionTransform = null,
+    Rect? ClientClipBounds = null,
+    double ClientClipCornerRadius = 0d);
+
+/// <summary>
+/// 区域点击穿透服务接口。
+/// </summary>
+public interface IRegionPassthroughService
+{
+    void SetInteractiveRegions(Window window, IReadOnlyList<WindowInteractiveRegion> interactiveRegions);
+    void ClearInteractiveRegions(Window window);
+    bool IsRegionPassthroughSupported { get; }
+}
+
+/// <summary>
+/// 无操作实现：非 Windows 平台窗口置底不可用。
+/// </summary>
+public sealed class NullWindowBottomMostService : IWindowBottomMostService
+{
+    public bool IsBottomMostSupported => false;
+
+    public void SetupBottomMost(Window window) { }
+
+    public void SendToBottom(Window window) { }
+
+    public PixelPoint GetScreenPosition(Window window) => window.Position;
+
+    public bool SetScreenPosition(Window window, PixelPoint position, bool queueOnFailure = false)
+    {
+        window.Position = position;
+        return true;
+    }
+}
+
+/// <summary>
+/// 无操作实现：非 Windows 平台区域穿透不可用。
+/// </summary>
+public sealed class NullRegionPassthroughService : IRegionPassthroughService
+{
+    public bool IsRegionPassthroughSupported => false;
+
+    public void SetInteractiveRegions(Window window, IReadOnlyList<WindowInteractiveRegion> interactiveRegions) { }
+
+    public void ClearInteractiveRegions(Window window) { }
+}