💥 Remove the legacy circuit surface for v4

[simon1hofmann]

🤖 AI text below 🤖

Description

Remove the complete legacy circuit surface for MQT Core v4: qc::QuantumComputation, CoreIR/CoreQASM targets and headers, mqt.core.ir, mqt.core.load, classic Qiskit converters, and circuit-oriented DD adapters. MQT Core v4 deliberately provides no compatibility aliases, legacy circuit constructs, or density-simulation layer.

CoreDD retains only its DD-native gate, control, qubit, and permutation vocabulary. The compiler-backed DDSIM QDMI provider accepts OpenQASM through QC/QCO and QIR. Terminal sampling simulates once and draws every shot from the resulting state; statevector extraction supports programs with only terminal measurements or no measurements. Expected compiler and simulation failures become failed jobs without exception-driven control flow.

When MLIR is disabled, MQT Core does not build the DDSIM device because no execution frontend remains. CoreDD, the QDMI driver, the superconducting device, and their applicable tests remain available.

This PR is the second and final layer of #2079 → #2288. The stack keeps the small math execution extensions and intentionally does not include density simulation. Symbolic Python execution can follow in a later release.

The local C++ lint wrapper now prints Clang diagnostics as file annotations instead of reporting only their count.

This implementation and PR description were produced with GPT-5.6 via Codex under explicit maintainer authorization. Maintainer review remains required.

Validation

• uvx nox -s lint
• uvx nox -s cpp-lint -- 5ebdb6da5da533b40ccdf19a5ab6af4e0a294d49 (Clang 22; zero findings)
• uvx nox -s tests-3.13 (745 passed, one Python 3.14-only test skipped)
• cmake --build --preset debug -j2
• ctest --preset debug --output-on-failure -j2 (3,755 passed)
• cmake --build --preset debug-no-mlir -j2
• ctest --preset debug-no-mlir --output-on-failure -j2 (511 passed, one intentional device-specification skip)
• The no-MLIR Ninja target graph contains no DDSIM device target.
• git diff --check

Fixes #2097
Fixes #2103

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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[burgholzer]

Approving this now to get it in once CI is green! 🚀
Issues for all kinds of follow-ups have been created.
So let's ship this

[simon1hofmann]

Seems like the maximum diff for linting is 20000 lines 🤔

[burgholzer]

Seems like the maximum diff for linting is 20000 lines 🤔

Yeah. We can simply ignore the failing check here and simply merge if everything else is green.

[simon1hofmann]

Seems like the maximum diff for linting is 20000 lines 🤔

Yeah. We can simply ignore the failing check here and simply merge if everything else is green.

Would be ready to merge now.

[burgholzer]

Let's go 🚀