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.

Cargo.lock

@@ -55,6 +55,7 @@ name = "display-qemu"
 version = "0.1.0"
 dependencies = [
  "display-server",
+ "driver-framebuffer-mock",
  "driver-framebuffer-virtio",
  "port-framebuffer",
  "server-framework",
@@ -283,6 +284,8 @@ dependencies = [
 name = "snake"
 version = "0.1.0"
 dependencies = [
+ "display-server",
+ "server-framework",
  "types",
  "userspace",
  "userspace-rt",