OnNewPSKReady could be applied out of order: two exchanges for a peer can derive
concurrently (one over signal, one over the data path), and the callbacks run
outside the manager lock, so an older exchange's apply could land after a newer
one and restore a stale WireGuard PSK, splitting the tunnel.
Give each exchange a per-peer monotonic generation, assigned under the lock at
creation so a later exchange always carries a higher one, and pass it to
OnNewPSKReady. The host adapter records the newest generation applied per peer
and drops any callback that is not newer, with the check-and-record atomic so the
slow SetPresharedKey call stays off that lock.
Found in cubic review on #7098 (client/internal/pqkem/callbacks.go:12).
riccardom
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