Native Wayland support via GTK4 + layer-shell

Summary

Implement native Wayland support using GTK4 and gtk4-layer-shell for proper overlay window positioning on Wayland compositors.

Current State

• Basic GTK4 + layer-shell integration
• Sprite rendering with Cairo
• Window positioning via layer margins
• Proper transparency (CSS approach may not work on all compositors)
• Click-through behavior verification
• Drag to pick up pet
• Screen size detection from GDK monitors

Architecture

Wayland compositor
    └── gtk4-layer-shell (overlay layer)
        └── GtkWindow (transparent)
            └── GtkDrawingArea
                └── Cairo (sprite rendering)

Dependencies

• gtk4
• gtk4-layer-shell

References

• https://github.com/wmww/gtk4-layer-shell
• https://wayland.app/protocols/wlr-layer-shell-unstable-v1