fix: remove broken QEMU GPU swap hack, add graceful fallback

VirtIO GPU:
- Removed QEMU 11.x command type swap hack (0x0104↔0x0106).
  The hack was wrong — swapping types without swapping data formats
  meant handlers read wrong struct layouts. Commands returned OK
  but did nothing.
- Restored correct VirtIO spec command types.
- Removed broken test pattern (fill + flush that never displayed).
- GPU setup now reports success/failure without crashing.

Display server resilience:
- VirtioFramebuffer::new() → try_new() -> Option<Self>.
  Returns None if phys-info page is unmapped or has zero dimensions.
- bins/display-qemu: tries VirtioFramebuffer, falls back to
  MockFramebuffer. Display server runs headless instead of crashing.
- No more page faults when GPU is absent.

The GPU driver code is architecturally correct (proper VirtIO
commands, proper data formats) but doesn't produce visible output
on QEMU 11.x due to a suspected QEMU legacy MMIO bug where
transfer_to_host_2d succeeds but doesn't transfer data. The driver
is ready for when this is fixed or a different QEMU version is used.

crates/bins/display-qemu/Cargo.toml

@@ -15,4 +15,5 @@ userspace = { workspace = true }
 server-framework = { workspace = true }
 display-server = { workspace = true }
 driver-framebuffer-virtio = { workspace = true }
+driver-framebuffer-mock = { workspace = true }
 port-framebuffer = { workspace = true }

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

@@ -3,14 +3,23 @@
 
 extern crate userspace_rt;
 
-use driver_framebuffer_virtio::VirtioFramebuffer;
 use types::bounded::CapSlot;
 
 #[unsafe(no_mangle)]
 extern "C" fn main() {
-    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);
+    let ep = CapSlot::const_new(0);
+
+    if let Some(fb) = driver_framebuffer_virtio::VirtioFramebuffer::try_new() {
+        let mut srv = display_server::server::DisplayServer::new(fb);
+        server_framework::serve(&mut srv, ep);
+    } else {
+        let fb = driver_framebuffer_mock::MockFramebuffer::new(
+            types::display::Dimension::new(640),
+            types::display::Dimension::new(480),
+        );
+        let mut srv = display_server::server::DisplayServer::new(fb);
+        server_framework::serve(&mut srv, ep);
+    }
+
     loop { userspace::syscall::yield_now(); }
 }