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/adapter-riscv/src/boot/loader.rs

@@ -161,10 +161,10 @@ fn load_elf(
         }
     }
 
-    for i in 0..4usize {
+    for i in 0..16usize {
         let sp = mmu::allocator::alloc_zeroed_page();
         mmu::map(user_pt, mmu::Mapping {
-            va: types::address::VirtualAddress::from_raw(USER_VA_BASE - (4 - i) * 4096),
+            va: types::address::VirtualAddress::from_raw(USER_VA_BASE - (16 - i) * 4096),
             pa: types::address::PhysicalAddress::from_raw(sp),
             pages: 1,
             flags: mmu::Sv39Flags::user_rw(),