fix+test: Euclidean holonomy/τ end-to-end + spherical edge-DOF oracle (2026-05-29 audit)

Bundles the 2026-05-29 Java↔C++ math-correctness audit (doc/reviewer/
java-port-audit.md, 11 findings) with two follow-up fixes.

Audit code changes:
- Finding 3 (spherical_functional): edge-DOF replacement parameterization via
  spher_eff_lambda; edge gradient α_opp⁺+α_opp⁻−θ_e (drops additive −(S⁺+S⁻)/2)
- Finding 4 (spherical_hessian): always-compiled edge-DOF throw guard
- Finding 6 (period_matrix): faithful normalizeModulus (0≤Re≤½, Im≥0, |τ|≥1)
- Finding 9 (inversive_distance): degenerate-face limiting angles, no skip
- Findings 1/2 (euclidean): degenerate gradient limiting angles + Hessian guard

Euclidean holonomy/τ fix: develop the cut surface across the dual spanning tree
only (cut_graph now exposes is_dual_tree), so genus-1 cut edges yield
non-degenerate lattice generators. Previously τ came out 0 / NaN / 1e13 on the
bundled tori; now matches the analytic revolution modulus i·√(R²−r²)/r. Re-enabled
τ reporting in the Euclidean CLI; rewrote validation.md §3/§4 accordingly.

Tests (240 CGAL, 0 skipped):
- HolonomyEndToEnd ×3 — tori of revolution (4×4, hex 6×6, 8×8) vs analytic modulus
- SphericalFunctional.EdgeGradient_RegularTetClosedForm — independent closed-form
  π/3 oracle locking the Finding-3 edge formula (the path-integral FD check cannot
  detect a wrong-but-conservative gradient)

Also documents the latent spherical/hyperbolic holonomy-extraction bug (same
single-development pattern, dead code today) in research-track.md (Phase 9c/10),
and adds favour/normalisations to the codespell ignore list.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

code/include/euclidean_hessian.hpp

@@ -44,6 +44,7 @@
 #include <Eigen/Sparse>
 #include <vector>
 #include <cmath>
+#include <stdexcept>
 
 namespace conformallab {
 
@@ -104,6 +105,18 @@ inline Eigen::SparseMatrix<double> euclidean_hessian(
 {
     const int n = euclidean_dimension(mesh, m);
 
+    // Only the vertex block of the cyclic Hessian is implemented here.  If any
+    // edge DOF is variable (assign_euclidean_all_dof_indices), the edge-edge and
+    // vertex-edge blocks present in the Java reference (conformalHessian) are
+    // missing, which would leave singular zero rows/cols.  Fail loudly instead
+    // of silently returning a rank-deficient matrix (matches the doc contract).
+    for (auto e : mesh.edges()) {
+        if (m.e_idx[e] >= 0)
+            throw std::logic_error(
+                "euclidean_hessian: edge DOFs are not supported "
+                "(only the vertex-block cotangent Laplacian is implemented)");
+    }
+
     // Collect triplets (row, col, value) — setFromTriplets sums duplicates.
     std::vector<Eigen::Triplet<double>> trips;
     trips.reserve(static_cast<std::size_t>(n) * 7);   // rough estimate