Prepare Moodle 5.3 beta on PHP 8.4 and exercise persistent LTS upgrades

[erseco]

Depends on #170; consumes ateeducacion/moodle#7. Builds the exact 5.3 beta/RC on PHP 8.4 from current main code. Stable/default promotion is separate in draft #172; 4.5 retains PHP 8.3.

Adds runtime and prerelease-version checks, 64-bit publication targets, three-backend beta CI and a real persistent 4.5 → 5.3 upgrade fixture. Integration exposed and fixed two defects: the PHP 8.4 base's Nginx fallback spelling, and retired bundled plugins (including qtype_random and nested subplugins) being preserved as custom plugins. Sync now uses Moodle's own deleted-plugin manifest.

The beta's actual environment.xml requires PostgreSQL 17 and MariaDB 11.4, newer than the public release notes. Tests pin those minimum series.

Validation completed locally on arm64 / Docker 29.7.2 / PHP 8.4.21:

• PostgreSQL 17, MariaDB 11.4 and SQLite: install, HTTP, runtime/extensions/iconv, Moosh, status, sync and restart passed.
• Real 4.5.14/PHP 8.3 → 5.3 beta/PHP 8.4 upgrade on PostgreSQL 17: course, file and installed plugin preserved; cron and subsequent restart passed (exit 0).
• 10 release-policy cases, 10 runtime cases, 14 sync scenarios, 13 version checks, 60 blueprint assertions, actionlint, targeted ShellCheck, PHP lint and diff checks passed.
• Real image gate accepts 5.3beta and rejects a mismatched 5.2beta label.

Full report, commands, rollout and rollback. CI adds amd64 coverage; its current result appears in the checks below. Test volumes were removed. No production images were published; remaining 32-bit image support is intentionally not advertised. Final stable 5.3 qualification and base-image security maintenance remain release gates.