Rewrite only ubuntu: image references on Incus

[bgrewell]

Closes #91.

The problem

TranslateImage rewrote every reference carrying a remote into images:<remote>/<alias>/cloud:

return fmt.Sprintf("images:%s/%s/cloud", remote, alias)

That is correct for ubuntu: — Canonical's simplestreams endpoint does not serve Incus clients, so the reference has to be redirected to the linuxcontainers server. It is wrong for everything else:

Written	Became	Result
lxc:alpine/3.18	images:lxc/alpine/3.18/cloud	no such path on that server
local:my-golden-image	images:local/my-golden-image/cloud	points at a public remote instead of the local store
myremote:custom/thing	images:myremote/custom/thing/cloud	private remote silently replaced

The /cloud suffix is a variant preference, not a fact about the alias namespace, and most image families do not publish it. Rewriting the remote is a separate error from choosing a variant, and this did both at once.

The fix

Only ubuntu: is rewritten. Everything else passes through as written — images:, lxc:, private and self-hosted remotes, and references with no remote at all. LXD is unaffected, as before.

An explicit ubuntu:24.04/cloud is no longer doubled into .../cloud/cloud.

The bug was tested in

TestTranslateImage asserted the old behaviour:

name:     "Incus lxc remote translated",
image:    "lxc:alpine/3.18",
expected: "images:lxc/alpine/3.18/cloud",

So the defect had a passing test describing it. That case now asserts the reference is left alone, with a comment explaining why. New table-driven tests cover the full matrix: LXD passthrough, the ubuntu rewrite, every other remote untouched, and the no-double-variant case.

Note on the second symptom in the issue

The issue also recorded that an already-translated images:ubuntu/24.04/cloud failed against a live daemon with The requested image couldn't be found for fingerprint "ubuntu/24.04/cloud", even though the equivalent incus launch worked. That is not fixed here and I could not reproduce it as a DART bug: the reference passes through TranslateImage untouched in both the old and new code, and the failure comes from the image server's alias lookup. The only local daemon on this machine is snap LXD, so the reference resolves against images.linuxcontainers.org rather than an Incus remote — a different path than the one the issue describes. Worth keeping #91 open on that point, or splitting it out, since the translation half is now settled.

Build, vet, gofmt, and the full test suite are green; the docs site builds. The docs section describing the old rewrite is replaced by what it does now.