🚚 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 and 5 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.

If PR contains AI-assisted content:

• Any agent that created, edited, or submitted GitHub content was explicitly authorized for that scope, as required by our AI Usage Guidelines.
• Every agent-authored or agent-edited public text body begins with the visible disclosure 🤖 *AI text below* 🤖 (titles are exempt).
• I have disclosed AI assistance in the PR description.
• I confirm that I have personally reviewed and understood all AI-generated content, and accept full responsibility for it.

[codecov]

Codecov Report

❌ Patch coverage is 92.05426% with 82 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/DensityUniqueTable.cpp	52.8%	42 Missing ⚠️
src/DensityDDPackage.cpp	94.3%	15 Missing ⚠️
src/DensityNode.cpp	94.0%	11 Missing ⚠️
src/NoiseFunctionality.cpp	97.2%	7 Missing ⚠️
include/DensityComputeTable.hpp	95.4%	2 Missing ⚠️
include/DensityNode.hpp	97.2%	2 Missing ⚠️
include/StochasticNoiseOperationTable.hpp	90.9%	2 Missing ⚠️
include/DensityDDPackage.hpp	90.9%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[burgholzer]

This likely requires an upgrade guide note as well.

[denialhaag]

@burgholzer, some changes since you last checked this:

1. Added an entry to the upgrade guide.
2. Copied over all tests that were removed in MQT Core so that coverage should be happy now.
3. Removed dead code in d3b9c8f, all of which is part of the vendored code initially added in this PR. Is any of this needed, or can we do without it? If we want to keep it, I'd say we should at least cover it with tests. 🤔

Claude Code also built and tested against the merge commit for munich-quantum-toolkit/core#1466 to ensure this PR didn't miss anything.

Aside from the dead code point, I'd say we can merge this. 🤔

[burgholzer]

All good to me 👍🏻 dead code can stay dead ☠️