build: run-x86-64-gpu — the interactive twin of the display-native test

Boots the usual VGA/GOP floor plus a virtio-gpu-pci adapter, so the
device-manager stack spawns the driver and the compositor upgrades to the
native fenced-present backend interactively. QEMU shows one head per
adapter: the live output is the virtio-gpu console in the View menu (the
VGA head freezes at the moment of upgrade). 512M, matching display-native.

build.zig

@@ -850,6 +850,53 @@ pub fn build(b: *std.Build) void {
     const run_efi_step = b.step("run-x86-64", "Boot the x86-64 kernel in QEMU (UEFI/OVMF); serial0 is logged to zig-out/qemu-test/run-x86-64-serial0-<timestamp>.log");
     run_efi_step.dependOn(&run_efi.step);
 
+    // --- run-x86-64-gpu: the same boot plus a virtio-gpu adapter ---
+    // The VGA device still supplies the boot (GOP) framebuffer the compositor starts
+    // on; the virtio-gpu function is discovered by the device-manager stack, its
+    // driver announces a shared scanout, and the compositor upgrades off the GOP
+    // floor to fenced — vsync, tear-free — native presents (docs/display-v2.md).
+    // This is the interactive twin of the `display-native` test case, and 512M
+    // matches it (the whole driver stack + the compositor's surfaces at once).
+    // QEMU shows one head per adapter: pick the virtio-gpu head in the View menu
+    // to watch the native output.
+    const run_gpu = b.addSystemCommand(&.{
+        "qemu-system-x86_64",
+        "-device",
+        "qemu-xhci,id=xhci",
+        "-device",
+        "usb-mouse,bus=xhci.0",
+        "-device",
+        "usb-kbd,bus=xhci.0",
+        "-machine",
+        "q35",
+        "-m",
+        "512M",
+        "-drive",
+        b.fmt("if=pflash,format=raw,readonly=on,file={s}", .{ovmf_code}),
+    });
+    run_gpu.addArg("-drive");
+    run_gpu.addPrefixedFileArg("if=pflash,format=raw,file=", vars_out);
+    run_gpu.addArg("-drive");
+    run_gpu.addPrefixedFileArg("if=none,id=bootusb,format=raw,file=", fat_image_serial);
+    run_gpu.addArgs(&.{
+        "-device",
+        "usb-storage,bus=xhci.0,drive=bootusb,removable=on,bootindex=0",
+        "-net",
+        "none",
+        "-vga",
+        "none",
+        "-device",
+        "VGA,edid=on,xres=1280,yres=720",
+        "-device",
+        "virtio-gpu-pci",
+    });
+    const gpu_serial_log = b.fmt("{s}/run-x86-64-gpu-serial0-{s}.log", .{ log_dir, timestamp(b) });
+    run_gpu.addArgs(&.{ "-serial", b.fmt("file:{s}", .{gpu_serial_log}) });
+    run_gpu.step.dependOn(&make_log_dir.step);
+
+    const run_gpu_step = b.step("run-x86-64-gpu", "Boot in QEMU with a virtio-gpu adapter: the compositor upgrades to fenced (tear-free) native presents; watch the virtio-gpu head in QEMU's View menu");
+    run_gpu_step.dependOn(&run_gpu.step);
+
     // const run_cmd = b.addRunArtifact(exe);
     // const run_step = b.step("run", "Run the app");
     // run_step.dependOn(&run_cmd.step);