Stop drained the data-path receive loop but not the host-driven signalling calls
(SignalOffer / SignalOnOffer / SignalOnAnswer). One in flight during shutdown
could still invoke OnNewPSKReady or OnRekeyFailed after Stop returned, touching a
WireGuard interface the engine was tearing down.
Register each signalling call in the wait group through beginCall, which refuses
once rootCtx is cancelled, mirroring startExchangeLocked's guard so no Add races
Stop's Wait. Stop already cancels, drains the transport, then Waits, so the Wait
now also covers these calls before state is cleared.
Found in cubic review on #7098 (client/internal/pqkem/manager.go:298).
riccardom
de70516045