Function execute 

pub fn execute(spec_path: &Path) -> Result<()>

Execute a spawn spec. Never returns to the caller:

• Unix: execvp replaces the current process. On exec failure we print a diagnostic to stderr and exit(127) (“command not found” convention).
• Windows: spawns a child, waits, and exits with the child’s code. Spawn failures also exit 127.

The Result<()> return type exists only so the caller can surface pre-spawn errors (spec read, parse, chdir) through the normal error path; once execvp/spawn is attempted, the process exits directly.