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/drivers/framebuffer/virtio/src/lib.rs

@@ -54,19 +54,22 @@ pub struct VirtioFramebuffer {
 }
 
 impl VirtioFramebuffer {
-    pub fn new() -> Self {
+    pub fn try_new() -> Option<Self> {
         let phys = read_phys_info();
+        if phys.width == 0 || phys.height == 0 || phys.cmd_pa.raw() == 0 {
+            return None;
+        }
         let width = Dimension::new(phys.width);
         let height = Dimension::new(phys.height);
         let stride = phys.width as usize * FB_BPP;
-        Self {
+        Some(Self {
             mmio: GpuMmio,
             queue: VirtioQueue::new(QueueAddrs::from_base(VQUEUE_VA)),
             dma: GpuDmaAddrs { cmd_pa: phys.cmd_pa, cmd_va: VirtualAddress::from_raw(CMDBUF_VA), resp_pa: phys.resp_pa },
             width,
             height,
             stride,
-        }
+        })
     }
 }