As a lawyer in China, Lu Siwei belonged to a rare and increasingly besieged group willing to take on sensitive cases to defend rights activists and political pariahs. To stop him, the authorities put him under surveillance and barred him from practice, depriving him of his livelihood. Mr. Lu’s wife and young daughter fled first, moving to the United States. Nearly two years later, it was Mr. Lu’s turn. He left China last month, crossing over into Laos. A few days later, as he was preparing to board a train…