Spring Productivity Rate Makes Small Gain WASHINGTON - Workers' productivity increased at an annual rate of 2.5 percent in the spring, the smallest gain since late 2002, reflecting the economy's slowdown.    The increase in productivity - the amount an employee produces for every hour on the job - was down from an initial estimate of a 2.9 percent growth rate for the April-to-June quarter and marked a deceleration from the 3.7 percent pace posted in the first three months of this year, the Labor Department reported Thursday...