Add Ubuntu 26.04 virtual machine

Add run-ubuntu2604.py with its QEMU config files and NoCloud seed. The
launcher follows the Debian 13 one, with UBUNTU_VM_ settings and SSH
forwarded to host port 2225. The seed injects the current user's SSH public
key, uses the USTC mirror (ubuntu-ports on arm64) and installs the GNOME
desktop on first boot. Update vm-images for the ubuntu2604 image target.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kwxp2XWvKnuEBifnCbhyX5

README.md

@@ -6,6 +6,7 @@ Start the required operating system with its distribution-specific launcher:
 ./run-debian13.py
 ./run-almalinux10.py
 ./run-fedora44.py
+./run-ubuntu2604.py
 ./run-windows11.py
 ```
 
@@ -46,8 +47,8 @@ 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 and Fedora, 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, and the VirtIO tools ISO
+for Windows 11 (whose installation ISO and base QCOW2 are supplied manually):
 
 ```bash
 ./run-fedora44.py --download-only
@@ -126,6 +127,35 @@ architecture-specific devices in `qemu/fedora44-amd64.ini` and
 `qemu/fedora44-arm64.ini`. Other Fedora settings use the `FEDORA_VM_` prefix.
 Run `./run-fedora44.py --help` for the complete list of overrides.
 
+## Ubuntu 26.04
+
+Start an Ubuntu 26.04 LTS (Resolute Raccoon) cloud image VM with:
+
+```bash
+./run-ubuntu2604.py
+```
+
+The launcher downloads the `ubuntu2604` image through the shared helper when
+it is missing, creates `.vm/ubuntu2604/disk.qcow2`, and builds its NoCloud seed
+from `seediso/ubuntu2604/`. Like the Debian seed, it injects the current user's
+SSH public key and points APT at the USTC mirror (`ubuntu-ports` on arm64).
+SSH is forwarded to host port `2225` by default:
+
+```bash
+ssh -p 2225 guofang@localhost
+```
+
+On the first boot, cloud-init installs `ubuntu-desktop-minimal` (GNOME) and
+`ibus-table-wubi`, makes the graphical target the default and reboots the
+guest. Keep the VM running until this initial installation and reboot have
+completed.
+
+The VM hardware (8G memory, 8 CPUs, drives, SSH port `2225`) is defined in
+`qemu/ubuntu2604.ini`, with the machine type and architecture-specific devices
+in `qemu/ubuntu2604-amd64.ini` and `qemu/ubuntu2604-arm64.ini`. Other Ubuntu
+settings use the `UBUNTU_VM_` prefix. Run `./run-ubuntu2604.py --help` for the
+complete list of overrides.
+
 ## Windows 11 23H2
 
 On first boot, the Windows 11 launcher creates a writable system-disk overlay