🚚 Vendor density-matrix DD support

[denialhaag]

Description

munich-quantum-toolkit/core#1466 has removed density-matrix DD support in MQT Core. The changes will be released as part of version 4.0.0. This PR moves the relevant code to MQT DDSIM.

AI notice: All changes are heavily assisted by Opus 4.8 via Claude Code.

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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[codecov]

Codecov Report

❌ Patch coverage is 87.36318% with 127 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/DensityUniqueTable.cpp	26.6%	66 Missing ⚠️
src/DensityDDPackage.cpp	90.5%	25 Missing ⚠️
src/DensityNode.cpp	88.7%	22 Missing ⚠️
src/NoiseFunctionality.cpp	97.2%	7 Missing ⚠️
include/DensityComputeTable.hpp	93.1%	3 Missing ⚠️
include/DensityNode.hpp	97.2%	2 Missing ⚠️
include/DensityDDPackage.hpp	90.9%	1 Missing ⚠️
include/StochasticNoiseOperationTable.hpp	88.8%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[burgholzer]

This likely requires an upgrade guide note as well.