Define window protocol for client-compositor communication

[pbalduino]

Summary

Define the protocol for communication between GUI clients and the compositor, including surface creation, buffer attachment, damage reporting, and input delivery.

Background

Inspired by Wayland protocol but simplified for meniOS. Clients send requests, compositor sends events.

Goals

• Define protocol message format
• Surface lifecycle (create, attach, commit, destroy)
• Input event delivery
• Frame callbacks for vsync
• Simple and extensible design

Protocol Messages

Client → Compositor (Requests)

```c enum request_type { REQ_CREATE_SURFACE, REQ_DESTROY_SURFACE, REQ_ATTACH_BUFFER, REQ_DAMAGE, REQ_COMMIT, REQ_FRAME_CALLBACK };

struct request { uint32_t type; uint32_t surface_id; union { struct { // ATTACH_BUFFER int fd; // Shared memory fd int width, height; } attach; struct { // DAMAGE int x, y, width, height; } damage; }; }; ```

Compositor → Client (Events)

```c enum event_type { EVT_CONFIGURE, EVT_KEY, EVT_MOUSE_MOTION, EVT_MOUSE_BUTTON, EVT_FRAME_DONE };

struct event { uint32_t type; uint32_t surface_id; union { struct { // CONFIGURE int width, height; } configure; struct { // KEY uint32_t keycode; uint32_t state; // pressed/released } key; struct { // MOUSE int x, y; } mouse; }; }; ```

Timeline

Total: 2-3 weeks

Definition of Done

• [ ] Protocol specification documented
• [ ] Message encoding/decoding implemented
• [ ] Client library provided
• [ ] Example client works
• [ ] Protocol extensible

Dependencies

• #402: Unix sockets
• #401: Shared memory

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