SAN DIEGO — Imagine jumping from a plane at 1,800 feet and linking up with 100 people in a giant snowflake formation, then scooting away from traffic to open your parachute in time to land safely. That is what 100 to 110 senior skydivers are planning to do in early April at Skydive Perris 20 miles north of Temecula, California, in the hope of setting a world record. Ejecting from five planes, they will exceed 120 mph and have 60 seconds to find their assigned link-up partners and grab arms and legs for a historic moment. This feat takes cloudless skies and clear minds capable of spur-of-the-m…