fix(minor): all five MINOR findings — doc, accuracy, and DRY

MINOR-1 (spherical_functional.hpp:426)
  Wrong comment said "second derivative < 0 for a convex functional".
  The spherical energy is *concave* (NSD Hessian); the monotone-f argument
  applies to both convex and concave functionals equally.  Comment rewritten
  to explain the actual physics: increasing scale increases all angles and
  thus reduces Σ G_v.

MINOR-2 (spherical_functional.hpp:494-495)
  Forward finite difference O(ε) → central finite difference O(ε²):
    old: dft = (sum_Gv(t + fd_eps) - ft) / fd_eps
    new: dft = (sum_Gv(t + fd_eps) - sum_Gv(t - fd_eps)) / (2*fd_eps)
  Same cost when the extra sum_Gv(t - fd_eps) replaces the cached ft.

MINOR-3 (euclidean_functional.hpp, spherical_functional.hpp,
         inversive_distance_functional.hpp)
  New header gauss_legendre.hpp centralises the 10-point Gauss-Legendre
  nodes and weights (gl10_nodes() / gl10_weights()).  The three energy
  functions now use the shared accessors instead of duplicated local
  static arrays.

MINOR-4 (euclidean_functional.hpp, spherical_functional.hpp,
         hyper_ideal_functional.hpp, inversive_distance_functional.hpp)
  halfedge_to_index() centralised in conformal_mesh.hpp.  All four local
  aliases (eucl_hidx, spher_hidx, hidx, id_detail::hidx) now delegate to
  it as one-line wrappers; the aliases are kept for now to avoid a larger
  call-site churn, clearly documented as thin wrappers.

MINOR-5 (clausen.hpp:33-38)
  Added a comment above inits() explaining the intentional off-by-one
  return value and how it interacts with csevl() — matching the Java
  Clausen.inits() / csevl() contract.

277/277 CGAL + 26/26 pure-math tests pass, 0 failed.

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

code/include/clausen.hpp

@@ -30,6 +30,12 @@ inline double csevl(double x, const double* cs, int n) noexcept {
 
 // Count Chebyshev terms needed so truncation error <= eta.
 // Corresponds to Java Clausen.inits().
+//
+// Note on return value: the loop decrements n one extra time after the
+// stopping condition is met, so the returned value is one less than the
+// last index checked.  Callers pass the result directly to csevl() as
+// the term count, which evaluates terms [0, n-1] — this is intentional
+// and matches the Java Clausen.inits() / csevl() contract exactly.
 inline int inits(const double* series, int n, double eta) noexcept {
     double err = 0.0;
     while (err <= eta && n-- != 0) {