const LATITUDE_LIMIT: number
How close to a pole a stance may sit, radians.
Not a singularity but a sign flip: geodeticDirection takes cos(latitude),
which goes negative past ±90° and reflects the direction through the axis
onto the opposite meridian. A stance recorded at 91°N therefore stands at
89°N on the far side of the world while reporting 91°N, so the camera and
every reading taken from it describe different ground. The margin only has to
survive the arithmetic, which is why it is 1e-6 rather than the two degrees
PITCH_LIMIT needs for a roll singularity.