refactor DensePolynomial add

[tcoratger]

Description

closes: #XXXX

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• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[Copilot]

Pull Request Overview

This PR refactors the addition implementation for DensePolynomial by simplifying control flow with early returns and improving readability.

• Implements early returns for zero polynomials.
• Introduces a (longer, shorter) strategy to minimize redundancy when summing coefficients.
• Utilizes the cfg_iter_mut! macro for in-place coefficient updates.

Comments suppressed due to low confidence (1)

poly/src/polynomial/univariate/dense.rs:309

• Consider adding a comment that explains the behavior and purpose of the cfg_iter_mut! macro, so future maintainers can quickly understand its functionality.

        cfg_iter_mut!(result)