Handle exp explicitly in as_magnitude()

The new test actually passed without modifying the code.  However, that
might be implementation-dependent (presumably based on canonicalization
of the `ratio` template parameter), so I wanted a more obviously correct
implementation.

src/core/include/units/magnitude.h

@@ -333,7 +333,9 @@ constexpr auto operator/(Magnitude auto l, Magnitude auto r) { return l * pow<-1
 template<ratio R>
   requires (R.num > 0)
 constexpr Magnitude auto as_magnitude() {
-  return detail::prime_factorization_v<R.num> / detail::prime_factorization_v<R.den>;
+  return pow<R.exp>(detail::prime_factorization_v<10>)
+    * detail::prime_factorization_v<R.num>
+    / detail::prime_factorization_v<R.den>;
 }
 
 namespace detail