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

seediso/ubuntu2604/README.md

@@ -0,0 +1,21 @@
+# Ubuntu 26.04 seed.iso
+
+This directory contains the cloud-init NoCloud configuration used by
+`run-ubuntu2604.py`.
+
+1. Edit `user-data`, `meta-data`, and (if needed) `network-config`.
+2. Build the image:
+
+   ```bash
+   ./seediso/ubuntu2604/build-seed.sh
+   ```
+
+   To choose another output path:
+
+   ```bash
+   ./seediso/ubuntu2604/build-seed.sh /path/to/seed.iso
+   ```
+
+The script uses `cloud-localds` when available, then falls back to `xorriso`,
+`genisoimage`, or `mkisofs`. Attach the resulting ISO as a CD-ROM when booting
+a cloud-init-enabled virtual machine.