♻️ Move OpenQASM serialization out of operations

[denialhaag]

Description

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Backports the OpenQASM serializer refactor to v3.x:

• ♻️ Move OpenQASM serialization out of operations #2249

The refactor moves OpenQASM formatting out of the operation hierarchy and into one serializer while preserving the existing QuantumComputation export APIs and emitted OpenQASM.

This backport intentionally differs from the original implementation:

• It exposes qasm3::Serializer from qasm3/Serializer.hpp, next to qasm3::Importer, instead of exposing qc::OpenQASMSerializer from ir/OpenQASMSerializer.hpp.
• It moves the serialization-specific register-map aliases from qc to qasm3.
• It keeps the implementation in CoreIR because CoreQASM already depends on CoreIR; moving the implementation between targets would create a dependency cycle or change the established CoreIR link contract.
• It updates the v3.x changelog, upgrade guide, internal calls, and tests for the adapted API and installed path.
• It omits the upstream internal implementation plan.

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.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[denialhaag]

@burgholzer and @simon1hofmann, now that we definitely keep this around for a while, I tried moving the serializer into the qasm3 subpackage. For more information, see the PR description. Let me know what you think! 🙂

[burgholzer]

I like it 👍🏻