perf(jacobian-action): major updates to jacobian action application by removing redudant quadrature work. ~5x increase in speed

libmeanfield/interface/field/field_registry.cppm

@@ -208,6 +208,9 @@ export namespace mean_field::field {
         using FormList = TypeList<Form::MeshExtension, Form::GravityForce, Form::CentrifugalForce, Form::ErrorNorm>;
     };
 
+    // Current realization of MultiplierFor<FixedTotalMass>. This remains a
+    // barotrope-specific field representation: the specification compiler,
+    // rather than the universal state registry, decides when it is present.
     struct BarotropicConstant {
         static constexpr std::string_view name = "barotropic_constant";
 
@@ -226,6 +229,26 @@ export namespace mean_field::field {
         static_assert(constraintsAreValid);
     };
 
+    // Solver border generated by FixedCentralDensity. This is deliberately a
+    // non-spatial numerical coordinate rather than a physical stellar field.
+    struct CentralDensityBorder {
+        static constexpr std::string_view name = "central_density_border";
+
+        using Support                          = NonSpatialSupport;
+
+        struct Scalar final : GlobalScalarQ {
+            static constexpr std::string_view symbol = "lambda_rho_c";
+        };
+
+        using Quantities                          = TypeList<Scalar>;
+        using Constraints                         = TypeList<>;
+        using FormList                            = TypeList<>;
+
+        static constexpr bool constraintsAreValid = validate_constraints(Constraints{});
+
+        static_assert(constraintsAreValid);
+    };
+
     // =========================================================================
     // Specific enthalpy
     //