وَجَاءَ رَبُّكَ
And your Lord comes.
One rhetorical and grammatical analysis offered here supplies the underlying expression “جَاءَ أَمْرُ رَبِّكَ”: the command of your Lord comes. In that reconstruction, “amr” is the possessed noun and “rabbika” is its genitive complement. Once “amr” is omitted, the expressed “rabbuka” occupies the subject position.
The point illustrated here is a particular mechanism of ellipsis and case change. A grammatical analysis of the wording should not be treated as a full account of all theological and exegetical questions surrounding the verse.