virtio-gpu: a modern virtio 1.0 display driver, bring-up to a flushed frame (v2 V3)

The first native scanout backend's driver half. The device manager matches the
virtio-gpu PCI function by its Display/Other class triple and spawns the driver,
which claims the device, confirms vendor 0x1AF4/device 0x1050 from config space,
enables memory-space + bus mastering, and walks the virtio vendor capabilities to
find the common-config and notify structures in its BAR.

From there it is the standard modern-virtio bring-up: reset, negotiate VERSION_1,
stand up the control virtqueue in coherent DMA, then drive the GPU end to end —
RESOURCE_CREATE_2D, ATTACH_BACKING (a coherent DMA buffer; V4 swaps in the shm-
shared surface), SET_SCANOUT, paint a known pattern, TRANSFER_TO_HOST_2D,
RESOURCE_FLUSH, and wait for the device's used-ring ack. Reading the backing back
proves it is CPU-visible RAM; the ack proves the device consumed the frame —
together the automated stand-in for "it's on screen", no screenshot.

- system/drivers/virtio-gpu/: the driver, plus virtio-gpu-protocol.zig (control
  commands) and virtio-pci.zig (the 1.0 PCI transport + split-virtqueue), both with
  host-tested struct sizes.
- device-manager matches the display/other class triple to "virtio-gpu"; the driver
  self-confirms the vendor/device id, since the class alone cannot distinguish it.
- ServiceId.scanout (11): the driver registers it so the compositor finds it in V4.

Gate: python3 test/qemu_test.py virtio-gpu (QEMU -device virtio-gpu-pci) — the
device-manager stack discovers the function, the driver brings up a 640x480 scanout
and flushes a test pattern: "virtio-gpu: scanout 640x480 online" + "flush acked,
pixel check ok". host tests, display-service, shm, and device-list still pass.

Note: the pre-existing pci-scan case triple-faults on main (verified at 88ad432,
before this change); it is unrelated and tracked separately.

build.zig

@@ -468,6 +468,7 @@ pub fn build(b: *std.Build) void {
     const fat_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "fat", "system/services/fat/fat.zig");
     const display_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "display", "system/services/display/display.zig");
     const display_demo_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "display-demo", "system/services/display-demo/display-demo.zig");
+    const virtio_gpu_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "virtio-gpu", "system/drivers/virtio-gpu/virtio-gpu.zig");
     const shm_server_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "shm-server", "system/services/shm-server/shm-server.zig");
     const shm_client_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "shm-client", "system/services/shm-client/shm-client.zig");
     const fat_test_exe = addUserBinary(b, kernel_target, runtime_module, mmio_module, xkeyboard_config_module, acpi_ids_module, "fat-test", "system/services/fat/fat-test.zig");
@@ -541,6 +542,8 @@ pub fn build(b: *std.Build) void {
     mk_run.addFileArg(display_exe.getEmittedBin());
     mk_run.addArg("display-demo");
     mk_run.addFileArg(display_demo_exe.getEmittedBin());
+    mk_run.addArg("virtio-gpu");
+    mk_run.addFileArg(virtio_gpu_exe.getEmittedBin());
     mk_run.addArg("shm-server");
     mk_run.addFileArg(shm_server_exe.getEmittedBin());
     mk_run.addArg("shm-client");
@@ -781,6 +784,8 @@ pub fn build(b: *std.Build) void {
         "system/services/fat/engine.zig", // FAT read/write over a RAM-backed image
         "system/services/display/compositor.zig", // Rect math + fill/composite/blit-tile
         "system/services/display/protocol.zig", // pack(): native pixel encoding per format
+        "system/drivers/virtio-gpu/virtio-gpu-protocol.zig", // virtio-gpu command struct sizes
+        "system/drivers/virtio-gpu/virtio-pci.zig", // virtio 1.0 PCI transport struct sizes
     }) |root| {
         const mod_tests = b.addTest(.{
             .root_module = b.createModule(.{