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>

README.md

@@ -34,7 +34,16 @@ ctest --test-dir build -R "^cgal\." --output-on-failure
 
 # Full build with CLI + viewer (requires Wayland/X11 dev headers)
 cmake -S code -B build -DWITH_CGAL=ON && cmake --build build -j$(nproc)
-./bin/conformallab_core -i input.off -g euclidean -o layout.off -j result.json
+
+# Conformal flattening (Θ_v = 2π target). Closed meshes pin one vertex +
+# enforce Gauss-Bonnet; open meshes pin the boundary and flatten the interior.
+./bin/conformallab_core -i code/data/off/torus_8x8.off -g euclidean -v \
+    -o layout.off -j result.json
+#  topology: closed, free DOFs=63, genus=1
+#  Euclidean: converged=yes  iter=3  |grad|_inf≈5e-15
+./bin/conformallab_core -i code/data/obj/cathead.obj -g euclidean -v -o cat.off
+#  topology: open (boundary pinned), free DOFs=119
+#  Euclidean: converged=yes  iter=4
 
 # API documentation (requires doxygen: brew/apt install doxygen)
 cmake --build build --target doc