ATM Tmux — Thin async wrapper over the tmux CLI.
Provides the [TmuxClient] trait for tmux pane management, with a real
implementation ([RealTmuxClient]) that shells out to tmux via
tokio::process::Command, and a mock ([MockTmuxClient]) for testing.
All code follows the panic-free policy: no .unwrap(), .expect(),
panic!(), unreachable!(), todo!(), or direct indexing [i].