Support and test 26.04 hosts

Charms are starting to move their HEAD and edge tracks to `ubuntu@26.04`, and an environment concierge prepares today cannot deploy them. In a review programme that deployed 149 charms one at a time against concierge-built controllers, 10 charms had to be reviewed at an older revision for this reason alone, and for `tempo-operators` the only deployable track predated the coordinated-workers split, so what was reviewed was not really the charm any more. That will get worse every month rather than better.

We should make 26.04 a host concierge supports and tests. The testing half is straightforward and mostly mechanical: the spread lxd backend already takes a `BASE`, but `systems:` lists only `ubuntu-24.04`, and the GitHub CI jobs run on `ubuntu-24.04` runners, so nothing exercises a 26.04 host at all.

The part I am less sure about is whether that is sufficient, and it seems worth finding out before deciding how big this is:

* Whether a concierge-prepared 26.04 host actually gives a controller that will accept `ubuntu@26.04` charms, on both the k8s and the LXD providers, or whether that is gated by the Juju version and the channels the presets pin rather than by anything concierge does.
* Whether the LXD provider needs anything for 26.04 images, or whether it follows from the host.
* Whether we want 26.04 as an additional tested system or as the default, and what that means for the 24.04 coverage we have now.

The wider gap this sits in is substrate coverage generally - the same corpus is 92 of 136 pure k8s charms with no MAAS and no public cloud anywhere in it - but 26.04 is the part that is actively blocking work today, so it seems like the place to start.

Start with the spread LXD backend and its BASE and systems configuration, then inspect the GitHub CI jobs using ubuntu-24.04 runners. Determine whether 26.04 works for both Kubernetes and LXD providers and whether Juju channels or presets affect support. Done means 26.04 coverage is added with the intended 24.04 coverage and CI exercises it.

Written by the indexing model from the issue text.