Fix second derivative when using Halleys method for iteration by length

[stribor14]

No description provided.

[Copilot]

Pull Request Overview

This PR fixes the calculation of the second derivative in Halley's method for iterating by arc length. The fix corrects how the second derivative is computed when using the chain rule to differentiate the arc length function.

• Corrected the second derivative calculation from derivativeAt(2, t).norm() to dC.dot(dC2) / df
• Improved code clarity by using a struct instead of std::pair and renaming variables for better readability
• Updated parameter name from s to ds to better reflect it represents a distance offset

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/bezier.cpp	Fixed second derivative calculation in Halley's method and improved variable naming
include/Bezier/bezier.h	Updated function signature to match implementation parameter name change

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[tiborbs1]

Approved.