Last year, Keith Lockhart found a silver lining when CBS pulled the plug on the national broadcast of the Fourth of July Boston Pops Fireworks Spectacular: creative control. This week, Lockhart is asserting that control with the first-ever Pops-directed webcast of the Independence Day extravaganza. Locally, WBZ will carry the TV broadcast; globally, fans can catch the live stream on bostonpopsjuly4th.org or cbsboston.com.