🐛 Harden MLIR conversion pass boundaries

[burgholzer]

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Description

Make the module-wide MLIR conversion contract explicit and fail closed at two unsafe conversion boundaries.

• Anchor the CBit-to-MemRef, Jeff/QCO, QCO/QC, and QC/QIR passes on mlir::ModuleOp, then use the typed pass root directly.
• Validate Jeff entry-point metadata and report malformed attributes as pass diagnostics instead of terminating the process.
• Reject multi-block QIR Base entry functions before normalization can mutate unsupported input.
• Inline the now-infallible, one-call Jeff cleanup path found by the Ponytail re-audit.

No new dependencies or public API changes are introduced.

Codex materially assisted with implementation, review, validation, and this pull-request text.

Validation:

• uvx nox -s lint
• uvx nox -s cpp-lint
• cmake --build --preset release -j2
• ctest --preset release --output-on-failure (4,033 passed; one expected skip)

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 user-facing contract changed.)
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals. (Not applicable: internal hardening only.)
• I have added migration instructions to the upgrade guide (if needed). (Not 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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[burgholzer]

FYI: @munich-quantum-toolkit/mqt-cc some genuinely useful simplifications found while working on #2077