perf(math): optimize aliquot sum calculation

[oozan]

Description of Change

Improves AliquotSum from O(n) to O(sqrt(n)) by finding divisors in pairs.

The implementation avoids double-counting square roots and preserves the existing input validation.

Testing

• gofmt
• go test ./...

Checklist

• Added description of change
• Added tests, including a perfect-square case
• Updated complexity documentation
• PR title follows semantic commit guidelines
• Searched for related open pull requests

AI assistance was used to prepare this focused change. I reviewed the algorithm and tests.

[codacy-production]

Not up to standards ⛔

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