✨ Add quantum-input QFT adder benchmark

[denialhaag]

Description

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Add a structured quantum-input QFT adder benchmark based on Draper's addition circuit. Each instance prepares an n-qubit addend in the uniform superposition and an n-qubit accumulator in state |1>, then applies the exact no-swap QFT, the complete controlled-phase addition, and the inverse QFT. The result contains addend || sum, which lets the analytic reference check the addition correlation instead of an uninformative uniform marginal.

This PR is stacked on #2324.

AI notice

This PR and its contents were created with the assistance of GPT-5.6 Sol via Codex.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
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• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[codecov]

Codecov Report

❌ Patch coverage is 98.47328% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/bench/JSON.cpp	87.5%	2 Missing ⚠️

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