On darwin, listenIn encrypts into one SendBatch while a sender goroutine
writes the other (batch.SendPipeline). The sender takes whatever built up
during its last write, so writes overlap tun reads and packets queued
behind a write leave in one WriteBatch, which a connected socket sends as
one sendmsg_x. Two batches bound memory; a full producer waits for the
swap rather than dropping. One producer and one sender keep per-peer order
equal to message-counter order.
Everywhere else the branch is a false constant and listenIn is unchanged.
-tags send_pipeline builds the pipeline on any platform, so its tests and
the e2e suite can run through it on linux.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
