- netrelay: only propagate CloseWrite on clean io.EOF; cancel both sides
on any other copy error so a short write, reset, or broken pipe can't
leave the opposite direction blocked.
- TestTCPCapPrefersTombstonedForEviction: assert both live pre-cap
entries survive, not just that the tombstone is gone, so a regression
that evicts a live entry instead of the tombstone is caught.