feat(libmeanfield): variadic refactor

also added normaliztion operator

libmeanfield/interface/field/field_registry.cppm

@@ -231,6 +231,28 @@ export namespace mean_field::field {
         static_assert(constraintsAreValid);
     };
 
+    // Scalar angular speed generated by FixedAngularMomentum. The axis and
+    // center belong to the compiled invariant, so the nonlinear coordinate
+    // contains only the signed speed along that fixed unit axis.
+    struct AngularVelocity {
+        static constexpr std::string_view name = "angular_velocity";
+
+        using PhysicalQuantity                 = dimensions::quantity::AngularVelocity;
+        using Support                          = NonSpatialSupport;
+
+        struct Scalar final : GlobalScalarQ {
+            static constexpr std::string_view symbol = "Omega";
+        };
+
+        using Quantities                          = TypeList<Scalar>;
+        using Constraints                         = TypeList<>;
+        using FormList                            = TypeList<>;
+
+        static constexpr bool constraintsAreValid = validate_constraints(Constraints{});
+
+        static_assert(constraintsAreValid);
+    };
+
     // Solver border generated by FixedCentralDensity. This is deliberately a
     // non-spatial numerical coordinate rather than a physical stellar field.
     struct CentralDensityBorder {