Function init 

pub fn init(log_dir: &Path, log_level: &str, file_log_level: &str) -> Result<()>

Initialize the process-wide tracing subscriber and log file sink. The function is idempotent: the first successful call installs the global subscriber, creates the log directory if needed, and keeps the file writer guard alive for the remainder of the process. Subsequent calls are no-ops. log_level controls the console filter, while file_log_level controls the file sink. Both are parsed through EnvFilter, so the configuration accepts standard tracing filter syntax rather than a bespoke log-level enum.