fix(eigen): validate basis operands before dense dispatch

src/fesa/analysis/eigen_reduction.cpp

@@ -118,6 +118,22 @@ Result<NodalLift> MakeLift(const Matrix& mass,
   if (positive == 0U) return Result<NodalLift>::Success(std::move(result));
 
   const Matrix scaled = ScaledMass(mass, length);
+  // Raw frame tangents can enter this internal boundary independently of
+  // Domain validation. Check the actual operands before any dense dispatch.
+  for (std::size_t row = 0; row < 6U; ++row) {
+    for (std::size_t j = 0; j < basis.Columns(); ++j) {
+      if (!std::isfinite(basis(row, j)))
+        return Result<NodalLift>::Failure(
+            Failure("invalid-eigen-mass-space", node,
+                    "Invalid positive mass basis."));
+    }
+    for (std::size_t j = 0; j < 6U; ++j) {
+      if (!std::isfinite(scaled(row, j)))
+        return Result<NodalLift>::Failure(
+            Failure("invalid-eigen-mass-space", node,
+                    "Scaled mass is nonfinite."));
+    }
+  }
   Matrix mp = dense_blas_internal::MultiplyTransposeLeft(basis, scaled)
                   .Multiply(basis);
   double scale = 0.0;