Implement window manager core

[pbalduino]

Summary

Implement the window manager that handles window placement, sizing, stacking order, and user interactions (move, resize, minimize, maximize, close).

Goals

• Window placement policies
• Move and resize operations
• Minimize, maximize, close
• Z-order management (raise, lower)
• Window decorations integration

Implementation

Window Operations

```c
void wm_move_window(struct window *w, int x, int y);
void wm_resize_window(struct window *w, int width, int height);
void wm_raise_window(struct window *w); // Bring to front
void wm_lower_window(struct window *w); // Send to back
void wm_close_window(struct window *w);
void wm_minimize_window(struct window *w);
void wm_maximize_window(struct window *w);
```

Window Placement

```c
// Cascade: Each new window offset from previous
void wm_place_cascade(struct window *w) {
static int offset = 0;
w->x = 100 + offset;
w->y = 100 + offset;
offset = (offset + 30) % 200;
}

// Center: Place in screen center
void wm_place_center(struct window *w) {
w->x = (screen_width - w->width) / 2;
w->y = (screen_height - w->height) / 2;
}
```

Timeline

Total: 3-4 weeks

Definition of Done

• Windows can be moved
• Windows can be resized
• Minimize/maximize/close work
• Z-order management works
• Placement policies work
• Smooth user experience

Dependencies

• Implement compositor core #403: Compositor core
• Implement GUI input event system #399: Input events

See docs/road/road_to_gui.md for complete roadmap.