Fix getHexagonRequiredNodeId2 formulas for even-k rings

[Copilot]

Two arithmetic bugs in ShapeGenerator::getHexagonRequiredNodeId2 produce wrong node IDs for rings with even k ≥ 4, breaking hexagon topology at larger sizes.

Changes

• Side 3, even k: (pos - 2) → ((pos - 2) / 2) — missing halving caused values to decrease twice as fast as intended
• Side 4, even k ≥ 4: 3*k*k - 6*k + 2 - pos → 3*k*k - 7*k + 6 - pos — wrong coefficients introduced an offset that grows with ring size

// Side 3, even k
-  return 3 * k * k - 6 * k + 3 - (pos - 2);
+  return 3 * k * k - 6 * k + 3 - ((pos - 2) / 2);

// Side 4, even k >= 4
-  return 3 * k * k - 6 * k + 2 - pos;
+  return 3 * k * k - 7 * k + 6 - pos;

Both bugs are latent for k=4 (only side 3 pos=2 is hit, where (pos-2)/2 == pos-2 == 0; side 4 formulas coincide when k=4) and first manifest at k=6. Verified against the full 169-value expected sequence and generateHexagon_61cells_withSeparation.