feat: implement phase 1 solver baseline

src/fesa.cpp

@@ -0,0 +1,8 @@
+#include "fesa/fesa.hpp"
+
+namespace fesa {
+static_assert(sizeof(GlobalId) == 8, "GlobalId must remain 64-bit");
+static_assert(sizeof(EquationId) == 8, "EquationId must remain 64-bit");
+static_assert(sizeof(SparseIndex) == 8, "SparseIndex must remain 64-bit");
+static_assert(sizeof(Real) == 8, "Real must remain double precision");
+}  // namespace fesa