scripts/dc-mirror.sh's generated sync script gains jammy-backports in --dist=,
with a comment at the generator recording why it is not optional: the MAAS
jammy image ships backports in its sources.list and juju's apt-mirror
model-config rewrites every suite to this mirror, so a mirror without it 404s
and every charm install hook fails. That cost 22 units today.

FIXED AT SOURCE, not only on the live rack -- a live-only edit would leave the
generator regenerating the old scope at the next install or at any future DC
standup.

tests/dc-mirror T10 asserted the old dist string verbatim. RE-POINTED to the
new invariant with the reason recorded in-file, never deleted to go green.
Mutation-proven: reverting the generator's dist line turns T10 red (23/1);
restored byte-identical, 24/24.

Gauntlet ALL GREEN (93); repo-lint 0 fail.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HvCyrwvYTTcDYnRErfMsNf