Fix ShapeGenerator::getHexagonAngle index calculation and side 2 pattern

[Copilot]

getHexagonAngle produced incorrect angles for hexagon construction due to wrong indexing and a flipped pattern on side 2.

Changes

• Index calculation: Removed the --n decrement that shifted to 0-based indexing. Now uses 1-based formula firstIndex = 1 + 3*(k-1)*k and ring-finding starts at k=1 with 1 + 3*k*(k+1) <= n.
• Side 2, odd k: Was [-120, +60, alternating...] — corrected to [alternating -120/+120..., +60, +120] where the special values are at positions k-2 and k-1 (end of side), not positions 0 and 1 (start).

Before (side 2, odd k):

if (pos == 0) return -120.0f;
if (pos == 1) return  60.0f;
return (pos % 2 == 0) ? 120.0f : -120.0f;

After:

if (pos == k - 2) return  60.0f;
if (pos == k - 1) return 120.0f;
return (pos % 2 == 0) ? -120.0f : 120.0f;