feat(libmeanfield): variadic refactor

also added normaliztion operator

extension_example/rotating_stellar_model.cppm

@@ -0,0 +1,61 @@
+module;
+
+#include <array>
+#include <utility>
+
+export module mean_field_extension_example.rotating_stellar_model;
+
+export import mean_field_extension_example.ideal_gas_radiation;
+import mean_field;
+
+/*
+ * This file is the physics-facing composition layer.  It contains no block
+ * matrices, generated residual types, Jacobian indices, or preconditioner
+ * plumbing.  StellarModel infers those structural types from the four
+ * physical specifications passed to it.
+ */
+export namespace mean_field::extension_example {
+    struct RotatingStellarModelParameters final {
+        IdealGasRadiation::Parameters equationOfState;
+        mean_field::dimensions::PressureValue surfacePressure;
+        mean_field::dimensions::MassValue totalMass;
+        mean_field::dimensions::AngularMomentumValue totalAngularMomentum;
+        std::array<double, 3> rotationAxis{0.0, 0.0, 1.0};
+        std::array<double, 3> rotationCenter{0.0, 0.0, 0.0};
+    };
+
+    [[nodiscard]] auto makeRotatingStellarModel(const RotatingStellarModelParameters &parameters) {
+        return mean_field::model::StellarModel(
+            IdealGasRadiation(parameters.equationOfState),
+            mean_field::surface::Isobaric({.Psurf = parameters.surfacePressure}),
+            mean_field::integral::FixedTotalMass({.Mtotal = parameters.totalMass}),
+            mean_field::integral::FixedAngularMomentum({
+                .Jtotal = parameters.totalAngularMomentum,
+                .axis = parameters.rotationAxis,
+                .center = parameters.rotationCenter
+            })
+        );
+    }
+
+    using RotatingStellarModel = decltype(
+        makeRotatingStellarModel(std::declval<const RotatingStellarModelParameters &>())
+    );
+
+    static_assert(mean_field::model::StellarModelType<RotatingStellarModel>);
+    static_assert(RotatingStellarModel::symbolicallySquare);
+
+    /*
+     * Deliberate capability boundary:
+     *
+     * The specification above is a valid, strongly typed stellar model.  The
+     * current numerical equilibrium core, however, closes density through a
+     * barotropic relation rho(h).  This EOS instead needs an independent
+     * temperature or entropy field and its governing equation.  Keeping this
+     * assertion false prevents an example from suggesting that discretize()
+     * already implements thermal equilibrium when it does not.
+     */
+    inline constexpr bool currentEquilibriumBackendSupportsIdealGasRadiation =
+        mean_field::equilibrium::StellarEquilibriumModel<RotatingStellarModel>;
+
+    static_assert(!currentEquilibriumBackendSupportsIdealGasRadiation);
+} // namespace mean_field::extension_example