feat: playable snake game via UART terminal rendering

Snake renders using ANSI escape codes via debug_putchar syscall,
bypassing the VirtIO GPU (which has a QEMU 11.x legacy MMIO bug
where transfer_to_host_2d succeeds but doesn't transfer data).

Full microkernel stack running end-to-end:
  adapter → init → storage-server IPC → load snake from disk →
  input-server IPC (UART keyboard) → snake game logic →
  debug_putchar syscall → UART → terminal display

Controls: WASD to steer, runs at 0.2s tick rate.
Green O=head, o=body, red @=food, #=border.

crates/bins/display-qemu/src/main.rs

@@ -8,20 +8,7 @@ use types::bounded::CapSlot;
 
 #[unsafe(no_mangle)]
 extern "C" fn main() {
-    let mut fb = VirtioFramebuffer::new();
-
-    use port_framebuffer::Framebuffer;
-    use types::display::{Color, Dimension, ScreenCoord};
-
-    // Initial clear to dark blue so we know the GPU works
-    fb.clear(Color::rgb(0, 0, 40));
-    fb.fill_rect(
-        ScreenCoord::new(10), ScreenCoord::new(10),
-        Dimension::new(200), Dimension::new(30),
-        Color::rgb(255, 255, 255),
-    );
-    fb.flush();
-
+    let fb = VirtioFramebuffer::new();
     let mut srv = display_server::server::DisplayServer::new(fb);
     let ep = CapSlot::new(0).unwrap();
     server_framework::serve(&mut srv, ep);