feat(macos): add traffic light position customization

[Illustar0]

What does the pull request do?

Adds support for customizing macOS traffic light button positions through MacOSProperties.TrafficLightPosition, including XAML usage.

API

/// <summary>
/// Defines the <c>TrafficLightPosition</c> attached property.
/// </summary>
public static readonly AttachedProperty<Point?> TrafficLightPositionProperty;

/// <summary>
/// Gets the top-leading position of the native macOS window buttons.
/// </summary>
/// <param name="window">The window.</param>
/// <returns>The position in logical units, or <see langword="null"/> when using the system layout.</returns>
public static Point? GetTrafficLightPosition(Window window);

/// <summary>
/// Sets the top-leading position of the native macOS window buttons.
/// </summary>
/// <param name="window">The window.</param>
/// <param name="value">The position in logical units, or <see langword="null"/> to use the system layout.</param>
public static void SetTrafficLightPosition(Window window, Point? value);

Usage

<Window
    ExtendClientAreaToDecorationsHint="True"
    WindowDecorations="Full"
    MacOSProperties.TrafficLightPosition="14,18" />

What is the current behavior?

The macOS traffic light buttons always use the system-provided position. Applications extending content into the title bar cannot align them with a custom title bar layout.

What is the updated/expected behavior with this PR?

Applications can specify the top-leading traffic light position in logical units. Setting the property to null restores the system layout.

How was the solution implemented (if it's not obvious)?

The native implementation keeps explicit System and Custom layout states. It preserves AppKit’s button sizes, spacing, RTL ordering, and tracking areas, while restoring system-owned layout for fullscreen and unsupported decoration modes.

Custom positioning is reapplied after AppKit title-bar layout changes, including initial display, resizing, appearance changes, and window-state transitions.

Electron and Zed implementations were researched to account for relevant AppKit behavior and known edge cases.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None

Obsoletions / Deprecations

None

Fixed issues

Fixes #21119

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069330-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]