Addresses code review on b22a7bd: (1) a rejected handshake now returns
{ status: 'failed', error } instead of throwing, keeping the structured
contract; (2) the "background handshake" warn only fires when the
timeout arm wins — not when the handshake is itself the race winner by
rejection. Timing tests migrated to vi.useFakeTimers for determinism.
Regression test added for the handshake-wins-by-rejection case.