The day before Dominion Voting Systems' defamation trial against Fox News was set to begin in a Delaware courthouse, the Fox board of directors and top executives made a startling discovery that helped lead to the breaking point between the network and Tucker Carlson, one of its top stars.Private messages sent by Mr. Carlson that had been redacted in legal filings showed him making highly offensive and crude remarks that went beyond the inflammatory, often racist comments of his prime-time show and anything disclosed in the lead-up to the trial.