The responder configures WireGuard with endpoint=nil first, then a delayed
update (scheduleDelayedUpdate) applies the real endpoint after fallbackDelay.
It captured the preshared key at schedule time and re-applied it. With the
post-quantum exchange the PSK can change within that window (a fresher PSK
derived and applied via SetPresharedKey), so re-applying the captured one
reverted WireGuard to a key the remote peer no longer used — a mismatch that
stalled the handshake until the WGWatcher timeout forced a retry (~30s).
Pass a nil PSK in the delayed update so it only sets the endpoint and leaves
the current PSK in place; the latest SetPresharedKey wins.
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