Replace binary-exists check with .jmri_installed_version marker pattern.
- Reads marker on each run; skips install if version matches
- On version mismatch: stops JMRI, wipes /opt/JMRI, downloads new archive
- Separates build hash into jmri_build_hash var (templated into download URL)
- Config is preserved — lives in git-managed .jmri symlink
To upgrade: bump jmri_version + jmri_build_hash, re-run playbook.