DegenerateFlectorException
The rejection of components that make no mirror motion — a would-be Flector whose mirror line lies at infinity.
Thrown by the Flector constructor when the mirror's normal norm √(fx² + fy²) rounds to zero at the SCALE quantum: the mirror line lies at infinity, and reflecting across it is a point inversion — a proper motion, not the improper isometry a flector encodes — so no unitized flector exists.