[burgholzer]

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Description

Require LLVM/MLIR for every MQT Core source build, including embedded builds. Remove the no-MLIR option, presets, and CI row, and use LLVM's SHA256 and unreachable facilities in place of the private implementations. Benchmark case IDs retain their exact bytes. Reuse LLVM environment lookup, scope guards, and a borrowed callback for private DD vector traversal. Scope guards also restore DD printing precision on early returns. This extracts the dependency change from #2545 without changing its error-handling APIs or exception/RTTI policy.

Installed Development consumers discover a matching LLVM SDK and inherit its headers and libraries. Runtime-only consumers still need no SDK, including when Development artifacts are present in the same installation. Optional Development lookup remains optional. Statically linked LLVM symbols stay private to the native shared libraries.

Migration: Remove BUILD_MQT_CORE_MLIR=OFF and use the regular debug or release presets. All source builds require LLVM/MLIR 23.1 or newer; Development consumers need the LLVM major/minor version used to build Core. Replace calls to the removed qdmi::unreachable() helper with llvm_unreachable("reason") and include llvm/Support/ErrorHandling.h directly. Empty QDMI environment variables now count as set; unset them to enable fallback discovery. An empty JSON configuration is diagnosed. Python APIs are unchanged.

Validation on Linux ARM64:

• Release build with LTO: 4,005 native tests passed; one existing test skipped.
• Built and installed the wheel: all 1,009 focused QDMI, Qiskit, and DD Python tests passed. Stub generation passed with no generated API changes. Benchmark case-ID golden tests also pass.
• Relocated source-install consumers passed with GCC and Clang; the Clang probe used native archives without GCC LTO bitcode. Device-free add_subdirectory embedding passed.
• Runtime-only source and wheel consumers passed with LLVM discovery disabled. Optional Development succeeded without LLVM; required/default Development failed with the intended diagnostic.
• Shared-library export checks found no exported LLVM definitions in CoreBench or CoreQDMI; Core API exports remain available.
• Repository lint and whole-file C++ lint passed. Existing SHA256 case-ID golden tests pass unchanged.

A focused review also checked callback lifetimes, cleanup, and environment handling. Windows/macOS execution and hosted CI remain pending.

AI assistance: Codex implemented and reviewed this change under explicit authorization.

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.
• 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

❌ Patch coverage is 57.14286% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
include/mqt-core/qdmi/common/Common.hpp	0.0%	7 Missing ⚠️
src/qdmi/QDMI.cpp	66.6%	1 Missing ⚠️
src/qdmi/driver/DeviceRegistry.cpp	80.0%	1 Missing ⚠️

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