Function resolve_command_on_path 

pub fn resolve_command_on_path(cmd: &str) -> Option<PathBuf>

What: Resolve an executable on PATH or by explicit path (Unix).

Input:

• cmd: Program basename or path containing MAIN_SEPARATOR.

Output:

• Some(path) for the first executable match; otherwise None.

Details:

• Honour Unix permission bits so a non-executable file on PATH is not treated as a tool.