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

qemu/ubuntu2604.ini

@@ -0,0 +1,69 @@
+# QEMU configuration for the Ubuntu 26.04 virtual machine, shared by all
+# architectures. The machine type and architecture-specific devices are in
+# ubuntu2604-<arch>.ini, which QEMU reads after this file.
+#
+# run-ubuntu2604.py fills in the file path placeholders below, writes the result
+# to <state dir>/qemu.ini and passes it to QEMU with -readconfig. Options that
+# have no -readconfig equivalent (-cpu, -display, daemon mode) are still added
+# by the launcher.
+
+[name]
+  guest = "ubuntu2604"
+
+# Accelerators are tried in order: KVM on Linux, HVF on macOS, else emulation.
+[accel]
+  accel = "kvm"
+
+[accel]
+  accel = "hvf"
+
+[accel]
+  accel = "tcg"
+
+[memory]
+  size = "8G"
+
+[smp-opts]
+  cpus = "8"
+
+# UEFI firmware: read-only code and the per-VM variable store.
+[drive "pflash0"]
+  if = "pflash"
+  format = "raw"
+  readonly = "on"
+  file = "${uefi_code}"
+
+[drive "pflash1"]
+  if = "pflash"
+  format = "raw"
+  file = "${uefi_vars}"
+
+# Writable qcow2 overlay on top of the shared base image.
+[drive "disk0"]
+  if = "none"
+  format = "qcow2"
+  file = "${disk_image}"
+
+[device "disk0-dev"]
+  driver = "virtio-blk-pci"
+  drive = "disk0"
+
+# cloud-init seed ISO.
+[drive "seed0"]
+  if = "none"
+  format = "raw"
+  readonly = "on"
+  file = "${seed_image}"
+
+[device "seed0-dev"]
+  driver = "virtio-blk-pci"
+  drive = "seed0"
+
+# User-mode networking with guest SSH forwarded to the host.
+[netdev "net0"]
+  type = "user"
+  hostfwd = "tcp::2225-:22"
+
+[device "nic0"]
+  driver = "virtio-net-pci"
+  netdev = "net0"