fix(gradient-checks): use relative error in all FD check functions

Finding-E from doc/reviewer/external-audit-2026-05-30.md.
Scan also uncovered the same issue in inversive_distance_functional.hpp.

Three functions used absolute error `|analytic - fd| > tol` while the
rest of the library (euclidean_functional, spherical_functional,
euclidean_hessian, hyper_ideal_functional) all use relative error
`|analytic - fd| / max(1, |analytic|) > tol`.

Absolute error is too strict for large gradients (false failures) and
too lenient for small gradients.

Fixed:
  cp_euclidean_functional.hpp  gradient_check_cp_euclidean()
  cp_euclidean_functional.hpp  hessian_check_cp_euclidean()
  inversive_distance_functional.hpp  gradient_check_inversive_distance()

All three now use the relative criterion and accumulate all failures
before returning (ok=false instead of early return on first mismatch).
Default tol updated from 1e-6/1e-5 to 1e-4, matching the Java
FunctionalTest convention used by all other checks in the library.
Error message updated to print rel-err instead of raw diff.

266/266 CGAL tests pass, 0 failed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

code/include/cp_euclidean_functional.hpp

@@ -324,16 +324,19 @@ inline Eigen::SparseMatrix<double> cp_euclidean_hessian(const ConformalMesh&
     return H;
 }
 
-/// FD gradient check for the CP-Euclidean functional.  Mirrors the
-/// Java `FunctionalTest`; default `eps = 1e-5`, `tol = 1e-6`.
+/// FD gradient check for the CP-Euclidean functional (central differences).
+/// Uses the same **relative** error criterion as every other gradient check in
+/// this library: `|analytic − fd| / max(1, |analytic|) < tol`.
+/// Default `eps = 1e-5`, `tol = 1e-4` (matches Java `FunctionalTest`).
 inline bool gradient_check_cp_euclidean(const ConformalMesh&        mesh,
                                           const std::vector<double>&  x,
                                           const CPEuclideanMaps&      m,
                                           double eps = 1e-5,
-                                          double tol = 1e-6)
+                                          double tol = 1e-4)
 {
     auto G = cp_euclidean_gradient(mesh, x, m);
     const std::size_t n = G.size();
+    bool ok = true;
 
     for (std::size_t i = 0; i < n; ++i) {
         std::vector<double> xp = x, xm = x;
@@ -341,28 +344,33 @@ inline bool gradient_check_cp_euclidean(const ConformalMesh&        mesh,
         xm[i] -= eps;
         const double Ep = cp_euclidean_energy(mesh, xp, m);
         const double Em = cp_euclidean_energy(mesh, xm, m);
-        const double fd = (Ep - Em) / (2.0 * eps);
-        if (std::abs(G[i] - fd) > tol) {
+        const double fd    = (Ep - Em) / (2.0 * eps);
+        const double err   = std::abs(G[i] - fd);
+        const double scale = std::max(1.0, std::abs(G[i]));
+        if (err / scale > tol) {
             std::cerr << "[cp-euclidean] FD gradient mismatch at DOF " << i
                       << ": analytic=" << G[i]
                       << "  FD=" << fd
-                      << "  diff=" << (G[i] - fd) << "\n";
-            return false;
+                      << "  rel-err=" << (err / scale) << "\n";
+            ok = false;
         }
     }
-    return true;
+    return ok;
 }
 
 /// FD Hessian check for the CP-Euclidean functional.  Verifies analytic
 /// `H` column-by-column against `(G(x+εe_j) − G(x−εe_j)) / (2ε)`.
+/// Uses the same **relative** error criterion as `hessian_check_euclidean`:
+/// `|analytic − fd| / max(1, |analytic|) < tol`.
 inline bool hessian_check_cp_euclidean(const ConformalMesh&        mesh,
                                          const std::vector<double>&  x,
                                          const CPEuclideanMaps&      m,
                                          double eps = 1e-5,
-                                         double tol = 1e-5)
+                                         double tol = 1e-4)
 {
     const auto H = cp_euclidean_hessian(mesh, x, m);
     const int n = static_cast<int>(H.rows());
+    bool ok = true;
 
     for (int j = 0; j < n; ++j) {
         std::vector<double> xp = x, xm = x;
@@ -372,19 +380,21 @@ inline bool hessian_check_cp_euclidean(const ConformalMesh&        mesh,
         auto Gm = cp_euclidean_gradient(mesh, xm, m);
 
         for (int i = 0; i < n; ++i) {
-            double fd = (Gp[static_cast<std::size_t>(i)] - Gm[static_cast<std::size_t>(i)])
-                        / (2.0 * eps);
-            double an = H.coeff(i, j);
-            if (std::abs(an - fd) > tol) {
+            double fd    = (Gp[static_cast<std::size_t>(i)] - Gm[static_cast<std::size_t>(i)])
+                           / (2.0 * eps);
+            double an    = H.coeff(i, j);
+            double err   = std::abs(an - fd);
+            double scale = std::max(1.0, std::abs(an));
+            if (err / scale > tol) {
                 std::cerr << "[cp-euclidean] FD Hessian mismatch at ("
                           << i << "," << j << "): analytic=" << an
                           << "  FD=" << fd
-                          << "  diff=" << (an - fd) << "\n";
-                return false;
+                          << "  rel-err=" << (err / scale) << "\n";
+                ok = false;
             }
         }
     }
-    return true;
+    return ok;
 }
 
 } // namespace conformallab