Add Satterthwaite and Kenward-Roger fixed-effect tests

include/spectra_reml/fixed_effects.hpp

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "spectra_reml/types.hpp"
+
+#include <vector>
+
+namespace spectra::reml {
+
+struct FixedEffectHypothesis {
+    // Rows are restrictions and columns correspond to beta in design order.
+    ColMajorMatrix contrast;
+    // Empty means a zero right-hand side.
+    std::vector<double> rhs;
+};
+
+// Computes coefficient-wise tests and any supplied general linear hypotheses
+// at an already fitted REML solution.  Inputs must use the same (possibly GRM-
+// rotated) coordinate system and phenotype scale as the supplied fit.
+[[nodiscard]] FixedEffectInferenceResult infer_fixed_effects_spectral(
+    const std::vector<double>& y_star, const ColMajorMatrix& x_star,
+    const std::vector<double>& eigenvalues, const RemlResult& fit,
+    FixedEffectTestMethod method,
+    const std::vector<FixedEffectHypothesis>& hypotheses = {},
+    double rank_tolerance_relative = 1e-10,
+    double covariance_floor_relative = 1e-12);
+
+}  // namespace spectra::reml