Windows launcher only creates a differential disk; seed via Unattend.xml

run-windows11.py no longer installs Windows or touches ISO/img files: it
creates a QCOW2 overlay on the base image (made by vm-images/build-windows-base.sh)
and, when the disk is created, mounts seed.iso. Windows reads the Unattend.xml
in its root during the specialize and OOBE passes (host name, account, auto
logon, Remote Desktop) and Configure-Windows.ps1 then installs OpenSSH with the
host's key, turns off Windows Update and copies the installer scripts.

The VM hardware uses devices with inbox Windows drivers (IDE/NVMe, e1000e,
ramfb on arm64), since the base image has no VirtIO drivers. Autounattend.xml
moves to vm-images; the vm-images pointer is updated.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -48,8 +48,7 @@ corresponding `*_STATE_DIR` environment variable:
 
 With `--download-only`, a launcher only downloads the media it would fetch
 automatically and then exits without creating a VM disk or starting QEMU: the
-base image for Debian, AlmaLinux, Fedora and Ubuntu, and the VirtIO tools ISO
-for Windows 11 (whose installation ISO and base QCOW2 are supplied manually):
+base image for Debian, AlmaLinux, Fedora and Ubuntu:
 
 ```bash
 ./run-fedora44.py --download-only
@@ -165,43 +164,49 @@ from the shared Windows 11 QCOW2 base image:
 ./run-windows11.py
 ```
 
-It creates a persistent 64G disk at `vm/windows11/disk.qcow2`, automatically
-partitions it, completes OOBE, creates the local administrator `guofang` with
-the initial password `wang`, and enables Remote Desktop on
-`0.0.0.0:3389` (all host interfaces; connect to `localhost:3389` on the host itself). During the first login it also silently installs the VirtIO
-drivers (the guest tools installer on amd64, the Arm64 driver packages from the
-VirtIO ISO with `pnputil` on arm64), without which the guest has no network on
-arm64. The installer includes the VirtIO drivers, SPICE vdagent, and QEMU Guest
-Agent. The launcher exposes the vdagent channel for host/guest clipboard
-integration and the QEMU Guest Agent at `vm/windows11/qemu-guest-agent.sock`.
-Change the password after first login. The OpenSSH Server is installed during
-the first logon from the Win32-OpenSSH zip for the host architecture, which the
-seed ISO carries (downloaded by `vm-images/download-images.sh openssh-win`), so
-Windows Update is not needed. Connect with `ssh guofang@localhost -p 2222`; the
-seed ISO also carries the host's SSH public key (`$SSH_PUBLIC_KEY_FILE`, else the
-first of `~/.ssh/id_ed25519.pub`, `id_ecdsa.pub`, `id_rsa.pub`) for passwordless
-administrator login.
+It creates a persistent 64G disk at `vm/windows11/disk.qcow2` as a QCOW2
+overlay on the base image and nothing else: the launcher only creates the
+differential disk and starts QEMU. The base image is generic and holds nothing
+but Windows (see `vm-images/README.md`): no account, no SSH, no VirtIO or other
+hardware-specific drivers, no machine identity. Like a cloud image, each VM is
+configured on its first boot, from the seed ISO.
+
+Whenever the disk does not exist yet, the launcher also mounts the seed ISO
+`vm/windows11/seed.iso` (beside the disk; built from `seediso/windows11/` when
+missing or outdated, with the OpenSSH zip and the host's SSH public key,
+`$SSH_PUBLIC_KEY_FILE` or the first of `~/.ssh/id_ed25519.pub`, `id_ecdsa.pub`,
+`id_rsa.pub`). Windows reads the `Unattend.xml` in the root of that CD-ROM
+during its specialize and OOBE passes: host name `WIN11`, time zone, Remote
+Desktop on `0.0.0.0:3389` (all host interfaces; connect to `localhost:3389` on
+the host itself), the local administrator `guofang` (initial password `wang`;
+change it after first login) and automatic logon. At the first logon it runs
+`Configure-Windows.ps1` from the seed ISO, which installs the OpenSSH Server
+from the zip with the SSH key (connect with `ssh guofang@localhost -p 2222`),
+turns off Windows Update and copies the numbered installer scripts
+(`seediso/windows11/NN-*.bat` and `.ps1`) to the desktop folder `Tools`. Later
+starts do not mount the seed ISO.
+
+Because the image has no VirtIO drivers, the VM only uses devices that Windows
+has inbox drivers for: an IDE disk and e1000e NIC on amd64, an NVMe disk, e1000e
+NIC and ramfb display on arm64 (a fixed-size framebuffer there). The SPICE
+vdagent and QEMU Guest Agent come with the VirtIO guest tools, so host/guest
+clipboard integration and the guest agent are not available unless you install
+those tools in the VM yourself.
 The Windows VM hardware (8G memory, 8 CPUs, system disk, devices)
 is defined in `qemu/windows11/config.ini`, with the accelerators and machine type in
 `qemu/windows11/<os>-<arch>.ini`; the launcher renders the former to
 `vm/windows11/qemu.ini` and loads both with `-readconfig`. Only the network
-(see Port forwards) and the installation media are still set by the launcher. QEMU arguments after
+(see Port forwards) and the seed ISO are still set by the launcher. QEMU arguments after
 `--` override the config files, for example `./run-windows11.py -- -m 4G -smp 4`.
-The generated answer-file ISO is stored beside the disk as
-`vm/windows11/seed.iso`.
 
-To install from the Windows ISO instead, use `./run-windows11.py
---install-from-iso`. This choice only applies while creating a missing system
-disk; subsequent starts always boot the existing system disk normally. During
-ISO installation, the launcher resolves its tools ISO through the shared image
-helper and downloads it when missing. Override the base QCOW2, source ISO,
-tools ISO, RDP port, or SSH port with the `WIN11_VM_` environment
-variables shown by `./run-windows11.py --help`. For example, when the VirtIO ISO
-is elsewhere:
-
-```bash
-WIN11_VM_TOOLS_ISO=/custom/path/virtio-win.iso ./run-windows11.py --install-from-iso
-```
+The launcher has no installation options and stops with a hint if the base image
+is missing. Make the base image once with `vm-images/build-windows-base.sh`,
+which is self-contained in `vm-images/` (its own answer file, QEMU config and
+installation logic, written in bash): it installs Windows unattended from the
+ISO into a scratch disk, runs `sysprep` as its last step and stores the disk as
+the read-only base image. Override the base QCOW2, seed ISO, RDP port, or SSH
+port with the `WIN11_VM_` environment variables shown by
+`./run-windows11.py --help`.
 
 ## Port forwards