Add SphericalPoint<T> type for spherical coordinate system

Implements ISO spherical coordinates (radius, azimuth, inclination) with
Degree<T> storage consistent with Rotation3<T>. Includes factory methods
(OnEquator, FromElevation, NorthPole, SouthPole), rotation methods,
Vincenty angular distance, slerp/lerp, and conversions to/from Point3,
Vector3, and CylindricalPoint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rafalmaciag