Fix unit test registration and fitter order-1 bug

Register test_radiation_ss_model and test_radiation_ss_fitter in CMakeLists.txt (source files existed but were never wired into CTest).

Fix fitter rejecting order-1 fits - the loop started at order=2 and FitFromSVD explicitly rejected O<2, so rank-1 kernels (single exponential) always returned invalid. Allow order=1 in both paths.

Switch TwoModeExponential test from pointwise relative error to peak-normalized absolute error (relative error is a poor metric for decaying kernels - can blow up in the tail where |K| is small).

Author: dav-og

include/hydroc/radiation/radiation_types.h

@@ -92,7 +92,8 @@ struct StateSpaceOptions {
     /**
      * @brief Minimum R² (coefficient of determination) threshold for fit acceptance.
      * 
-     * The fitter adds modes until either R² >= r2_threshold or max_order is reached.
+     * The fitter tries all orders up to max_order and keeps the best R².
+     * Higher orders can add fast-decaying poles that improve transient accuracy.
      * Values closer to 1.0 require better fits but may need more modes.
      * Typical values: 0.90-0.99. Default: 0.95.
      */

src/hydro/radiation/radiation_ss_fitter.cpp

@@ -121,7 +121,7 @@ StateSpaceFitResult RadiationStateSpaceFitter::FitKernel(
     // enough" R² prevents these fast poles from getting through, causing a
     // characteristic slight lag after the first response lobe.
     // =========================================================================
-    for (int order = 2; order <= max_possible_order; ++order) {
+    for (int order = 1; order <= max_possible_order; ++order) {
         StateSpaceFitResult candidate = FitFromSVD(K, dt, order, y, U, V, svh, hankel_size);
 
         if (candidate.IsValid() && candidate.r2 > result.r2) {
@@ -150,7 +150,7 @@ StateSpaceFitResult RadiationStateSpaceFitter::FitFromSVD(
     const int N = static_cast<int>(K.size());
     const int O = order;
 
-    if (svh.size() < O || O < 2) {
+    if (svh.size() < O || O < 1) {
         return result;
     }
 

tests/unit/CMakeLists.txt

@@ -32,3 +32,5 @@ endfunction()
 # --- Register unit tests ---
 message(STATUS "Add HydroChrono unit tests")
 build_unit_test(test_added_mass_determinism)
+build_unit_test(test_radiation_ss_model)
+build_unit_test(test_radiation_ss_fitter)

tests/unit/test_radiation_ss_fitter.cpp

@@ -165,22 +165,20 @@ bool TestTwoModeExponential() {
     TEST_ASSERT(result.order >= 2, "Should find at least 2 modes");
     TEST_ASSERT(result.r2 >= 0.98, "R² should be >= 0.98");
 
-    // Verify reconstruction
+    // Verify reconstruction using peak-normalized absolute error.
+    // Relative error is a poor metric for decaying kernels (blows up in the tail).
     Eigen::VectorXd K_fit = RadiationStateSpaceFitter::ReconstructKernel(result, dt, N);
 
-    double max_rel_error = 0.0;
-    for (int k = 0; k < N; ++k) {
-        if (std::abs(K(k)) > 0.01) {
-            double rel_error = std::abs(K(k) - K_fit(k)) / std::abs(K(k));
-            max_rel_error = std::max(max_rel_error, rel_error);
-        }
-    }
-
-    TEST_ASSERT(max_rel_error < 0.1, "Max relative error should be < 10%");
+    const double K_peak = K.lpNorm<Eigen::Infinity>();
+    double max_abs_error = (K - K_fit).lpNorm<Eigen::Infinity>();
+    double normalized_error = max_abs_error / K_peak;
 
     std::cout << "    R² = " << std::fixed << std::setprecision(6) << result.r2 << std::endl;
     std::cout << "    Order = " << result.order << std::endl;
-    std::cout << "    Max relative error = " << max_rel_error << std::endl;
+    std::cout << "    Max abs error = " << max_abs_error 
+              << " (" << std::setprecision(2) << 100.0 * normalized_error << "% of peak)" << std::endl;
+
+    TEST_ASSERT(normalized_error < 0.05, "Peak-normalized error should be < 5%");
     std::cout << "  PASSED" << std::endl;
     return true;
 }