test: add unit test suite for quaternion module

quaternion/ has zero _test.v files despite a rich README documenting SLERP, NLERP, 3D rotations, interpolation, exponentiation, and parity operations.

Task

Add tests under quaternion/:

1. quaternion/rotation_test.v — verify rotation matrices, axis-angle conversions, to/from matrix
2. quaternion/interpolation_test.v — SLERP, NLERP with known reference values
3. quaternion/basic_test.v — arithmetic, identity, norm, normalize

Verification

• cd ~/.vmodules/vlang && v test vsl/quaternion
• All operations tested with tolerance 1e-6