Replace MP_UNITS_CONSTEVAL with constexpr in unit operations

Changes MP_UNITS_CONSTEVAL to constexpr for unit multiplication, division,
and inverse operations in unit.h. This allows these functions to be used
in runtime contexts, which is necessary for DSP applications where unit
calculations may depend on runtime parameters.

Specifically fixes compilation error with clang 21+ where consteval
functions cannot be called in non-constant contexts, such as:
```c++
  m_frequency = inverse<si::hertz>(period);
```

Author: rothmichaels

src/core/include/mp-units/framework/unit.h

@@ -147,7 +147,7 @@ struct unit_interface {
    * Multiplication by `1` returns the same unit, otherwise `scaled_unit` is being returned.
    */
   template<UnitMagnitude M, Unit U>
-  [[nodiscard]] friend MP_UNITS_CONSTEVAL Unit auto operator*(M, U u)
+  [[nodiscard]] friend constexpr Unit auto operator*(M, U u)
   {
     if constexpr (std::is_same_v<M, MP_UNITS_NONCONST_TYPE(mp_units::mag<1>)>)
       return u;
@@ -171,7 +171,7 @@ struct unit_interface {
    * Returns the result of multiplication with an inverse unit.
    */
   template<UnitMagnitude M, Unit U>
-  [[nodiscard]] friend MP_UNITS_CONSTEVAL Unit auto operator/(M mag, U u)
+  [[nodiscard]] friend constexpr Unit auto operator/(M mag, U u)
   {
     return mag * inverse(u);
   }
@@ -182,7 +182,7 @@ struct unit_interface {
    * to the derived unit and the magnitude remains outside forming another scaled unit as a result of the operation.
    */
   template<Unit Lhs, Unit Rhs>
-  [[nodiscard]] friend MP_UNITS_CONSTEVAL Unit auto operator*(Lhs lhs, Rhs rhs)
+  [[nodiscard]] friend constexpr Unit auto operator*(Lhs lhs, Rhs rhs)
   {
     return expr_multiply<derived_unit, struct one>(lhs, rhs);
   }
@@ -193,7 +193,7 @@ struct unit_interface {
    * to the derived unit and the magnitude remains outside forming another scaled unit as a result of the operation.
    */
   template<Unit Lhs, Unit Rhs>
-  [[nodiscard]] friend MP_UNITS_CONSTEVAL Unit auto operator/(Lhs lhs, Rhs rhs)
+  [[nodiscard]] friend constexpr Unit auto operator/(Lhs lhs, Rhs rhs)
   {
     return expr_divide<derived_unit, struct one>(lhs, rhs);
   }
@@ -593,7 +593,7 @@ template<Unit T, typename... Expr>
 
 MP_UNITS_EXPORT_BEGIN
 
-[[nodiscard]] MP_UNITS_CONSTEVAL Unit auto inverse(Unit auto u) { return one / u; }
+[[nodiscard]] constexpr Unit auto inverse(Unit auto u) { return one / u; }
 
 /**
  * @brief Computes the value of a unit raised to the `Num/Den` power