Answering a link card took one environment-wide lock and held it across
PeerLinks.link, which calls the other machine. A peer that was slow to answer
blocked every other card's answer until it timed out. The lock is now per
card, through the shared KeyedLock, which is all that keeps two answers to
one card from both linking.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>