Add a Rocky Linux 9 virtual machine

run-rocky9.py with its QEMU config, port forwards (SSH on 2226) and
cloud-init seed, modelled on the Fedora 44 launcher. Update vm-images for
the rocky9 download target.

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

seediso/rocky9/README.md

@@ -0,0 +1,17 @@
+# Rocky Linux 9 seed.iso
+
+This directory contains the cloud-init NoCloud configuration used by
+`run-rocky9.py`.
+
+`build-seed.sh` injects your SSH public key into `user-data` (the `@SSH_PUBKEY@`
+placeholder): `$SSH_PUBKEY_FILE`, else the first `~/.ssh/id_*.pub`, else the first
+key offered by `ssh-agent`.
+
+After changing `user-data`, `meta-data`, or `network-config`, remove the old
+`seed.iso` and rebuild it:
+
+```bash
+./seediso/rocky9/build-seed.sh
+```
+
+Use a unique `instance-id` when cloning this configuration for another VM.