runAndStreamAux funnels a command's stdout and stderr into a single
cmdWriter (the command-log panel, or a buffer when output is suppressed)
from two separate goroutines: stderr through the MultiWriter set on
cmd.Stderr, and stdout through the onRun callback. Those goroutines
wrote the shared writer without any synchronization, racing on the
prefixWriter's prefixWritten flag and interleaving the two streams.
Wrap the writer so its writes are serialized.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>