build: arch-aware mariadb.repo and lsphp index lookup (amd64 unchanged)

- Dockerfile, Dockerfile.fpm: select rhel$releasever-<amd64|aarch64> from uname -m
  (amd64 file is byte-identical; $basearch cannot be used, x86_64 path 404s).
- scripts/lsphp-repo-version.sh replaces three inline binary-amd64 lookups
  (build-push.yaml x2, lsphp-source-emergency.yaml); arch defaults to amd64.
- fixture test wired into Shell-Checks.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0185ukXjXyosSkunDsdWz3mb

Dockerfile.fpm

@@ -77,6 +77,8 @@ COPY ./configs/prod-php.ini /etc/php.ini
 # all images ship PHP >= 8.2.35 / 8.3.36 / 8.4.27 / 8.5.12.
 COPY ./configs/zz-php2shell-mitigation.ini /etc/php.d/zz-php2shell-mitigation.ini
 COPY ./configs/mariadb.repo /etc/yum.repos.d/
+# mirror.mariadb.org names the arch dir amd64/aarch64 (not $basearch); amd64 result is byte-identical.
+RUN sed -i "s/rhel\$releasever-amd64/rhel\$releasever-$(uname -m | sed s/x86_64/amd64/)/" /etc/yum.repos.d/mariadb.repo
 
 # Set up cron job for log rotation
 RUN echo "15 */12 * * * root /scripts/log-rotate.sh" >> /etc/crontab