start method
void
start()
Implementation
void start() {
if (_active) return;
_active = true;
if (!_ansiSupported) {
// CI mode: print once, no animation
stdout.writeln('$_label...');
return;
}
stdout.write(_Ansi.hideCursor);
_timer = Timer.periodic(interval, (_) => _render());
_render(); // draw first frame immediately
}