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/adapter-riscv/src/devices/gpu_setup.rs

@@ -53,28 +53,11 @@ pub fn configure(uart: &Uart, display_pid: ProcessId) {
     }
 
     gpu.init_device(vq_phys);
-    gpu.setup_framebuffer(cmd_phys, vq_phys, fb_phys, FB_WIDTH, FB_HEIGHT);
-
-    // Fill framebuffer with WHITE and flush — visible test pattern
-    let fb = fb_phys as *mut u32;
-    let total = FB_WIDTH.raw() as usize * FB_HEIGHT.raw() as usize;
-    for i in 0..total {
-        unsafe { core::ptr::write_volatile(fb.add(i), 0xFFFFFFFF); }
+    if gpu.setup_framebuffer(cmd_phys, vq_phys, fb_phys, FB_WIDTH, FB_HEIGHT) {
+        uart.write_str("k-os: VirtIO GPU configured\n");
+    } else {
+        uart.write_str("k-os: VirtIO GPU setup failed (display will be headless)\n");
     }
-
-    let queue = hw_virtio::queue::VirtioQueue::new(
-        hw_virtio::queue::QueueAddrs::from_base(vq_phys),
-    );
-    let dma = hw_virtio::gpu::GpuDmaAddrs {
-        cmd_pa: types::address::PhysicalAddress::from_raw(cmd_phys),
-        cmd_va: types::address::VirtualAddress::from_raw(cmd_phys),
-        resp_pa: types::address::PhysicalAddress::from_raw(cmd_phys + 512),
-    };
-    let mmio = hw_virtio::mmio::DirectMmio::new(gpu.base());
-    hw_virtio::gpu::transfer_to_host(&mmio, &queue, &dma, 1, 0, 0, FB_WIDTH.raw() as u32, FB_HEIGHT.raw() as u32);
-    hw_virtio::gpu::resource_flush(&mmio, &queue, &dma, 1, 0, 0, FB_WIDTH.raw() as u32, FB_HEIGHT.raw() as u32);
-
-    uart.write_str("k-os: VirtIO GPU configured\n");
 }
 
 fn map_page(pt: &mut mmu::PageTable, va: usize, pa: usize) {