feat(interop): add safe handle wrappers and configuration options

Implement SafeHandle wrappers for native resources (SafeGtkWidgetHandle, SafeGObjectHandle, SafeX11DisplayHandle, SafeX11CursorHandle, SafeDlopenHandle) to prevent memory leaks. Move gesture and rendering configuration from static properties to LinuxApplicationOptions for better testability and DI compatibility. Add THREADING.md and DI-MIGRATION.md documentation. Include LayoutIntegrationTests for Measure/Arrange pipeline and SkiaViewTheoryTests with parameterized test cases using [Theory] attributes.

Native/SafeHandles.cs

@@ -0,0 +1,196 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.InteropServices;
+using Microsoft.Win32.SafeHandles;
+
+namespace Microsoft.Maui.Platform.Linux.Native;
+
+/// <summary>
+/// Safe handle wrapper for GTK widget pointers.
+/// Releases the widget via <c>gtk_widget_destroy</c> when disposed.
+/// </summary>
+internal class SafeGtkWidgetHandle : SafeHandleZeroOrMinusOneIsInvalid
+{
+    [DllImport("libgtk-3.so.0")]
+    private static extern void gtk_widget_destroy(IntPtr widget);
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeGtkWidgetHandle"/> that owns the handle.
+    /// </summary>
+    public SafeGtkWidgetHandle() : base(ownsHandle: true)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeGtkWidgetHandle"/> wrapping an existing pointer.
+    /// </summary>
+    /// <param name="existingHandle">The existing GTK widget pointer.</param>
+    /// <param name="ownsHandle">Whether this safe handle is responsible for releasing the resource.</param>
+    public SafeGtkWidgetHandle(IntPtr existingHandle, bool ownsHandle = true) : base(ownsHandle)
+    {
+        SetHandle(existingHandle);
+    }
+
+    /// <inheritdoc />
+    protected override bool ReleaseHandle()
+    {
+        gtk_widget_destroy(handle);
+        return true;
+    }
+}
+
+/// <summary>
+/// Safe handle wrapper for GObject pointers.
+/// Releases the object via <c>g_object_unref</c> when disposed.
+/// Suitable for any GObject-derived type including GtkCssProvider, GdkPixbuf, etc.
+/// </summary>
+internal class SafeGObjectHandle : SafeHandleZeroOrMinusOneIsInvalid
+{
+    [DllImport("libgobject-2.0.so.0")]
+    private static extern void g_object_unref(IntPtr obj);
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeGObjectHandle"/> that owns the handle.
+    /// </summary>
+    public SafeGObjectHandle() : base(ownsHandle: true)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeGObjectHandle"/> wrapping an existing pointer.
+    /// </summary>
+    /// <param name="existingHandle">The existing GObject pointer.</param>
+    /// <param name="ownsHandle">Whether this safe handle is responsible for releasing the resource.</param>
+    public SafeGObjectHandle(IntPtr existingHandle, bool ownsHandle = true) : base(ownsHandle)
+    {
+        SetHandle(existingHandle);
+    }
+
+    /// <inheritdoc />
+    protected override bool ReleaseHandle()
+    {
+        g_object_unref(handle);
+        return true;
+    }
+}
+
+/// <summary>
+/// Safe handle wrapper for X11 <c>Display*</c> pointers.
+/// Releases the display connection via <c>XCloseDisplay</c> when disposed.
+/// </summary>
+internal class SafeX11DisplayHandle : SafeHandleZeroOrMinusOneIsInvalid
+{
+    [DllImport("libX11.so.6")]
+    private static extern int XCloseDisplay(IntPtr display);
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeX11DisplayHandle"/> that owns the handle.
+    /// </summary>
+    public SafeX11DisplayHandle() : base(ownsHandle: true)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeX11DisplayHandle"/> wrapping an existing pointer.
+    /// </summary>
+    /// <param name="existingHandle">The existing X11 Display pointer.</param>
+    /// <param name="ownsHandle">Whether this safe handle is responsible for releasing the resource.</param>
+    public SafeX11DisplayHandle(IntPtr existingHandle, bool ownsHandle = true) : base(ownsHandle)
+    {
+        SetHandle(existingHandle);
+    }
+
+    /// <inheritdoc />
+    protected override bool ReleaseHandle()
+    {
+        XCloseDisplay(handle);
+        return true;
+    }
+}
+
+/// <summary>
+/// Safe handle wrapper for X11 Cursor resources.
+/// Releases the cursor via <c>XFreeCursor</c> when disposed.
+/// Requires the associated <c>Display*</c> to be provided at construction time,
+/// as X11 cursor cleanup requires both the display and cursor handles.
+/// </summary>
+internal class SafeX11CursorHandle : SafeHandleZeroOrMinusOneIsInvalid
+{
+    [DllImport("libX11.so.6")]
+    private static extern int XFreeCursor(IntPtr display, IntPtr cursor);
+
+    private readonly IntPtr _display;
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeX11CursorHandle"/> that owns the handle.
+    /// </summary>
+    /// <param name="display">
+    /// The X11 Display pointer required for releasing the cursor.
+    /// The caller must ensure the display remains valid for the lifetime of this handle.
+    /// </param>
+    public SafeX11CursorHandle(IntPtr display) : base(ownsHandle: true)
+    {
+        _display = display;
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeX11CursorHandle"/> wrapping an existing cursor.
+    /// </summary>
+    /// <param name="display">
+    /// The X11 Display pointer required for releasing the cursor.
+    /// The caller must ensure the display remains valid for the lifetime of this handle.
+    /// </param>
+    /// <param name="existingHandle">The existing X11 Cursor handle.</param>
+    /// <param name="ownsHandle">Whether this safe handle is responsible for releasing the resource.</param>
+    public SafeX11CursorHandle(IntPtr display, IntPtr existingHandle, bool ownsHandle = true) : base(ownsHandle)
+    {
+        _display = display;
+        SetHandle(existingHandle);
+    }
+
+    /// <inheritdoc />
+    protected override bool ReleaseHandle()
+    {
+        if (_display != IntPtr.Zero)
+        {
+            XFreeCursor(_display, handle);
+        }
+        return true;
+    }
+}
+
+/// <summary>
+/// Safe handle wrapper for <c>GtkCssProvider*</c> pointers.
+/// Since GtkCssProvider is a GObject, this releases it via <c>g_object_unref</c> when disposed.
+/// </summary>
+internal class SafeCssProviderHandle : SafeHandleZeroOrMinusOneIsInvalid
+{
+    [DllImport("libgobject-2.0.so.0")]
+    private static extern void g_object_unref(IntPtr obj);
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeCssProviderHandle"/> that owns the handle.
+    /// </summary>
+    public SafeCssProviderHandle() : base(ownsHandle: true)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new <see cref="SafeCssProviderHandle"/> wrapping an existing pointer.
+    /// </summary>
+    /// <param name="existingHandle">The existing GtkCssProvider pointer.</param>
+    /// <param name="ownsHandle">Whether this safe handle is responsible for releasing the resource.</param>
+    public SafeCssProviderHandle(IntPtr existingHandle, bool ownsHandle = true) : base(ownsHandle)
+    {
+        SetHandle(existingHandle);
+    }
+
+    /// <inheritdoc />
+    protected override bool ReleaseHandle()
+    {
+        g_object_unref(handle);
+        return true;
+    }
+}