Sizeof Dev and Test Sets (C3W1L06)

Picked one by one from a mixed channel. Transcript check: both title readers certain; 5 of 5 sampled certain picks confirmed by transcript, none failed. Channel rule: Include only actual lesson videos from the Coursera specializations (titles carrying course codes like C1W4L06 or '[Course N, Week N, Lesson N]') and long-form 'Full Course' videos; exclude course announcements, enroll ads, hackathon and winner clips, event recaps, conference talks and Heroes interviews.