Task: Implement Hotfix: Site build needs live GNU ELPA

The site package bootstrap must survive an elpa.gnu.org outage, and site CI must not install the packages on every run.

• A cold run with unreachable GNU archives installs all five site packages.
• A warm .packages tree makes ores-site-ensure-packages a no-op.
• Both site workflows restore and save ./.packages.

1. Add ores-site--archives-usable-p. It reads the refresh outcome from package-archive-contents, because package-refresh-contents only warns on a failed archive and never signals.
2. Add ores-site--refresh-once and ores-site--use-archive-mirrors.
3. Rewrite ores-site--refresh-with-retries to try the primary GNU archives first, then the mirrors.
4. Cache ./.packages in build-site.yml and site-cdash.yml.

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

Root cause: the site packages need queue and compat, which exist only on GNU ELPA. The bootstrap fetched them on every run with no CI cache, and package-refresh-contents only warns on a failed archive, so the three-attempt retry never ran and the queue guard failed on the first pass. An elpa.gnu.org outage therefore failed the build for its whole duration.

Fix: the refresh now reads its outcome from package-archive-contents and retries, then falls back to the BFSU mirrors. Both site workflows cache ./.packages, so a warm cache removes the network dependency entirely.

The mirror is a supply-chain input, so it matters that Emacs checks the archive signatures against the GNU keyring; a mirror cannot serve different content.

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

The bootstrap reads the refresh outcome, retries, and falls back to the BFSU mirrors. Both site workflows cache ./.packages. A cold run with unreachable GNU archives installed all five packages; a warm run was a no-op. Merging PR #2693 makes site CI survive the next GNU ELPA outage.