♻️ Migrate circuit flattening to Core v4

[simon1hofmann]

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Description

Prepare the MQT Debugger for the removal of qc::CircuitOptimizer in MQT Core #2262:

• call QuantumComputation::flattenOperations(true) directly; and
• remove the obsolete optimizer header and CMake target.

This intentionally leaves the MQT Core version requirement and uv.lock unchanged. The PR must remain a draft until Core v4 is released, at which point the dependency pin can be updated and the PR revalidated.

Stack and dependencies

This is a stacked draft targeting codex/2098-openqasm-serializer and depends on #436, because current Debugger main still uses OpenQASM APIs already removed from Core v4. After #436 merges, retarget this PR to main. It also depends on Core #2262 and should merge only after the Core v4 release.

Companion migration drafts

• Core: munich-quantum-toolkit/core#2262
• QCEC: munich-quantum-toolkit/qcec#1042
• QMAP: munich-quantum-toolkit/qmap#1125
• QuSAT: munich-quantum-toolkit/qusat#514
• DDSIM: munich-quantum-toolkit/ddsim#977
• Debugger: munich-quantum-toolkit/debugger#438

Validation

• warning-clean build with WARNINGS_AS_ERRORS=ON
• complete 433-target build
• 149/149 full C++ tests
• full uvx nox -s lint
• git diff --check

AI assistance

Codex materially assisted with implementation, stacked-branch validation, review, and this pull request description. A human must review and understand the changes before marking this pull request ready.

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. (Not applicable: no public Debugger API changes.)
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks. Local Core v4 validation passes; CI requires the future Core v4 pin.
• I have reviewed my own code changes.

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