ST. MORITZ, Switzerland — Lindsey Vonn didn’t just pop into the hospital one day, get a new titanium knee and then decide on the way out that she wanted to return to downhill ski racing. It’s been a long, calculated process involving several minor and some major knee surgeries, careful vetting of the medical issues involved and then months of on-snow testing in New Zealand, Austria and Colorado to see how her body and new knee would react at age 40. So now that she’s preparing to step into a World Cup starting gate this weekend for the first time in nearly six years for super-G races in St.