perf: replace polling loop with event-driven waiting for process output

[Danthewaann]

Replace the fixed time.sleep(0.1) in the main render loop with an event-driven wait using selectors. Each process's stdout file descriptor is registered with a selector, and the main loop blocks in select() until either new output is ready to read or a short timeout elapses, instead of redrawing on a fixed interval regardless of whether anything changed.

Process output is now read directly on the main thread instead of via a background reader thread per process, since the selector already tells us when there's data available. When a process exits, any output still sitting in its pipe is drained immediately so trailing output written right before exit isn't missed.

Also update the progress spinner so it's update interval is tied to the elapsed time for each process. This means it can still render at a consistent interval since the time.sleep(0.1) call is gone, which previously gated when it got updated.