Refine the requires-expressions of the `cartesian_vector` operators to
constrain on `const`-qualified operands, matching the fact that the
operators take their arguments by `const` reference. A type whose
`operator+` (etc.) is not const-correct would previously have wrongly
satisfied the constraint.
The compound-assignment constraints are ref-qualified as well (mutable
lhs, const rhs) while keeping the `-> std::same_as<T&>` return-type check.
Also strengthen the unit-vector test to assert the resulting component
values (not just that the norm is 1) and to exercise the free
`unit_vector()` function.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Mateusz Pusz