docs: Clarify that "box" means "axis-aligned bounding box"

Instead of "a generic collection of vertices to form a box".
tests: Increase the euler coverage
tests: Improve code coverage for quaternions
tests: Improve coverage for spheres
sphere: Use vec3.equal()

Comparing the SIMD4f directly does not take into account floating point
fluctuations. The equal() operator for vectors does a comparison and a
near() check for us.
box: Use vec3.equal()

Like we do for graphene_sphere_equal(). The direct SIMD4f comparison
operator ignores floating point fluctuations and can lead to false
negatives.