🐛 Initialize Qiskit classical bits in OpenQASM 3

[burgholzer]

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Description

Backport explicit classical-bit initialization from the v4 Qiskit QDMI backend.

Qiskit initializes classical bits to zero, while OpenQASM 3 bit declarations leave them uninitialized. Prepend a false store for every classical bit so partially measured circuits preserve their Qiskit semantics.

This extracts the focused fix and regression test from #2288 without backporting the unrelated v4 circuit-surface removal.

The v3 legacy importer now type-checks indexed assignment targets as scalar elements and accepts only leading false bit stores as the IR zero default; later classical computation remains rejected.

Validation

• uv run --no-sync pytest test/python/plugins/qiskit/test_mock_backend.py (21 passed)
• uv run --no-sync pytest test/python/plugins/qiskit/test_backend.py test/python/plugins/qiskit/test_estimator.py test/python/plugins/qiskit/test_sampler.py (65 passed)
• mqt-core-ir-test --gtest_filter=Qasm3ParserTest.* (104 passed)
• uvx nox -s lint

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.
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• The changes follow the project 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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[denialhaag]

LGTM!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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