The darwin tun returned one packet per Read, and listenIn flushes its
UDP send batch after every Read, so each outbound packet went to the
underlay as its own batch. tunQueue.Read now keeps calling readv until
the utun would block, up to tunReadBatch packets, so a burst reaches the
send batch together.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
