feat(euclidean): full analytic edge-DOF (cyclic) Hessian; Tier-2 Wente finding

Upgrades the cyclic Euclidean Hessian from block-FD to closed-form, satisfying
the novelty-statement §3.2 "analytic Hessians, not finite difference" claim for
the Euclidean path.

- euclidean_hessian.hpp: `euclidean_hessian_analytic` — closed-form cyclic
  Hessian from the law-of-cosines angle derivatives
    ∂α_i/∂s_i = ℓ_i²/4A,  ∂α_i/∂s_j = ½cot α_i − ℓ_j²/4A   (Σ_j = 0),
  chained to (u, λ_e) and sign-mapped to the gradient outputs (−α vertex,
  +α_opp edge). Reuses euclidean_cot_weights. Block-FD kept as cross-check.
- newton_solver.hpp: newton_euclidean cyclic path now uses the analytic Hessian.
- tests: CyclicHessian_Analytic_MatchesBlockFD_Tetrahedron — analytic == block-FD
  (1e-6), == gradient FD (1e-5), symmetric (1e-9). Existing cyclic convergence
  oracle still GREEN with the analytic Hessian routed in.

Tier-2 (Wente) finding: wente_torus02.obj is a QUAD mesh (1240 quads) and the
Java golden comes from cyclic (quad-net) uniformization; the C++ period-matrix
pipeline is triangle-based, so a faithful bit-vs-Java τ comparison needs a
quad/cyclic pipeline (Phase 9f). Deferred and documented; golden τ = ½+i√3/2.

244/244 cgal tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

code/include/newton_solver.hpp

@@ -258,13 +258,13 @@ inline NewtonResult newton_euclidean(
         }
 
         // ── Hessian + solve H·Δx = −G  (SparseQR fallback for singular H) ──
-        // Cyclic layout (edge DOFs present) → block-FD Hessian, which covers the
-        // vertex-edge / edge-edge blocks the analytic cotangent Laplacian omits.
-        // Vertex-only layout → analytic cotangent Laplacian (cheaper, exact).
+        // Cyclic layout (edge DOFs present) → analytic cyclic Hessian, covering
+        // the vertex-edge / edge-edge blocks the vertex-only cotangent Laplacian
+        // omits.  Vertex-only layout → analytic cotangent Laplacian.
         bool has_edge_dof = false;
         for (auto e : mesh.edges())
             if (m.e_idx[e] >= 0) { has_edge_dof = true; break; }
-        auto H = has_edge_dof ? euclidean_hessian_block_fd_sym(mesh, x, m)
+        auto H = has_edge_dof ? euclidean_hessian_analytic(mesh, x, m)
                               : euclidean_hessian(mesh, x, m);
         bool ok = false;
         Eigen::VectorXd dx = detail::solve_with_fallback(H, -G, ok);