Function resolve_log_dir 

pub fn resolve_log_dir() -> PathBuf

Resolve the benchmark log directory.

Mirrors benchmark_resolve_log_dir() from the shell script:

• Prefers MLSTACK_LOG_DIR environment variable
• Falls back to $HOME/.rusty-stack/logs
• Final fallback to ${TMPDIR:-/tmp}/rusty-stack/logs

Creates the directory if it doesn’t exist.