[uf] unite() の返り値の情報量を増やす

[ia7ck]

No description provided.

[Copilot]

Pull Request Overview

This PR modifies the unite() method in the Union-Find data structure to provide more detailed information about the union operation. Instead of returning a simple boolean, it now returns an Option<UniteResult> containing information about which node became the root and which became the child.

• Changed unite() return type from bool to Option<UniteResult>
• Added new UniteResult struct to capture union operation details
• Updated documentation and examples to reflect the API change

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