fix: VirtIO GPU driver and snake rendering through display server

- Fix swapped command IDs: attach_backing was 0x0104 (FLUSH),
  resource_flush was 0x0106 (ATTACH_BACKING). Now correct.
- Increase process stack from 4 pages (16 KB) to 16 pages (64 KB)
  to prevent stack overflow in display process.
- Restructure display-qemu to avoid MockFramebuffer's 32 KB array
  inflating the VirtIO path's stack frame.
- Wire snake app to render through display server via IPC instead
  of UART escape codes. Uses existing renderer.rs pixel renderer.
- Init now delegates display endpoint to snake process.

crates/init/src/lib.rs

@@ -165,11 +165,10 @@ pub fn run(shared_buf_va: usize) -> ! {
         debug_print(b")\n");
     }
 
-    // Load snake — give it input endpoint (slot 0) for keyboard input
-    // Snake renders via debug_putchar (UART) — no display endpoint needed
     debug_print(b"init: loading snake... ");
     if let Some(snake) = load_binary(shared_buf, b"snake.elf") {
         syscall::delegate(INPUT_EP, snake.process_cap, ipc_perms);
+        syscall::delegate(DISPLAY_EP, snake.process_cap, ipc_perms);
         syscall::spawn_thread_in(snake.process_cap, snake.entry_point, USER_STACK_TOP);
         debug_print(b"ok\n");
     } else {