docs(euclidean-hessian): fix wrong cotangent formula in two comment blocks

Finding-C from doc/reviewer/external-audit-2026-05-30.md.

The box comment at the top and the function-level comment above
euclidean_cot_weights() both stated:

  cot_k = (t_adj1·l123 − t_adj2·t_opp) / denom2   ← WRONG

The correct formula (verified numerically on a 3-4-5 right triangle,
expected cot1=4/3, cot2=3/4, cot3=0) is:

  cot_k = (t_opp · l123 − t_a · t_b) / denom2

where t_opp is the t-value of the edge OPPOSITE vertex k, and t_a/t_b
are the t-values of the two edges ADJACENT to vertex k.

The implementation in euclidean_cot_weights() was already correct;
only the documentation was wrong.

Changes (documentation only, zero code changes):
- Box comment: rewritten with correct formula and explicit per-vertex
  assignment (cot1: t_opp=t23, t_a=t12, t_b=t31; etc.)
- Box comment: added missing ½ factor to Hessian contribution lines
- Function-level comment: corrected to (t_opp·l123 − t_a·t_b)/(8·Area)
  with a pointer to the box comment for the full assignment
- Inline return comment in euclidean_cot_weights(): now shows the
  mapping (cot1: t_opp=t23, t_a=t12, t_b=t31) directly at the formula

263/263 CGAL tests pass, 0 failed.

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

doc/reviewer/external-audit-2026-05-30.md

@@ -701,7 +701,7 @@ index. Add a comment:
 |----|------|-------|------|----------|--------|
 | A | `hyper_ideal_functional.hpp` | 319–344 | Bug | Critical (mixed config) | ✅ Fixed 2026-05-30 |
 | B | `gauss_bonnet.hpp` | 87–88, 128–134 | API error | Medium | ✅ Fixed 2026-05-31 |
-| C | `euclidean_hessian.hpp` | 26–27, 57–58 | Doc error | Medium | 🟡 Open |
+| C | `euclidean_hessian.hpp` | 26–27, 57–58 | Doc error | Medium | ✅ Fixed 2026-05-31 |
 | D | `euclidean_functional.hpp` + 2 others | 97–107 | Doc error | Medium | 🟡 Open |
 | E | `cp_euclidean_functional.hpp` | 338–349, 373–387 | Inconsistency | Medium | 🟡 Open |
 | F | test files | — | Test gap | Medium | 🟠 Open |