Function handle_engine_command 

pub fn handle_engine_command(
    cmd: &str,
    arg: &str,
    runtime: &mut Runtime,
) -> Option<CommandResult>

Process commands that are pure engine logic — no TUI state needed. Returns None if the command needs TUI-level handling.

NOTE: this runs BEFORE any renderer-level command arms (the TUI calls it first and returns early on Some — see tui/commands.rs). Renderer arms for commands handled here are unreachable for the matched cases.