Function spawn_start 

pub fn spawn_start(opts: SpawnOptions) -> Result<RunningProcess, CoreError>

Start spawning a process and return a monitor handle.

This initializes the pipes and starts the process, but does not block. Use RunningProcess::register_with_reactor, RunningProcess::handle_reactor_event, RunningProcess::io_done, and RunningProcess::into_output_parts to drive captured stdio without exposing internal drain state.

Errors

• EACCES: Permission denied for the executable.
• EINVAL: Invalid spawn options (e.g. background capture without wait).
• EMFILE: Process limit on open file descriptors hit.
• ENOENT: The executable was not found.
• ENOMEM: Insufficient memory to spawn the process.