Enforce path traversal validation and deterministic user listing

[Cylae]

Enforced validate_safe_path in atomic_write, ProcessLock::acquire, and Journal::open_or_create to prevent path traversal vulnerabilities. Optimized and sorted user listing deterministically in UserManager::list_users. Added contract tests for all changes.

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PR created automatically by Jules for task 1430597682708282895 started by @Cylae

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