Today marks a milestone for thousands of Senior Executive Service members, general military officers and other top government officials: The onset of new reporting requirements for most stock purchases and other securities transactions worth more than $1,000. Up to now, such transactions had to be reported only once a year on the public financial disclosure statement known as Office of Government Ethics Form 278. Now, senior officials must file “transaction reports” within 30 to 45 days, under the recently passed STOCK Act. The new mandate applies to almost all 28,000 Form 278 filers, according to the ethics office, which issued…