A man pursued his ex-girlfriend into their daughter’s bedroom in Burlington last week and stabbed her 34 times, and the fatal attack was witnessed by someone whom the daughter had been videochatting with on an iPad, a prosecutor said today. The new details emerged at the Woburn District Court arraignment of Christopher Piantedosi, who faces a murder charge in the death Thursday of Kristen Pulisciano. Piantedosi was ordered held without bail, and a probable cause hearing in the case was slated for June 7. Prosecutors said the person on the other end of the videochat both saw and heard portions of the attack, including Pulisciano pleading, “Chris, please stop, I love you,” and Piantedosi saying, “You’re going to die.”