Update README to reflect full 2D/3D library capabilities

Comprehensive rewrite covering 3D types (Point3, Vector3, Matrix3x3,
Quaternion), coordinate systems (SphericalPoint, CylindricalPoint),
rotations (Rotation3, Euler/quaternion), poses and trajectories
(Pose3, PosePath3, Trajectory3), kinematics (Joints6), data structures
(KdTree, Skeleton), and pooled allocations.

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

Author: rafalmaciag