Cleanup D ownership

[melody-ren]

Per offline discussion, this line in particular is pointing at a bigger issue. Namely, the D matrix probably never should've been part of our standard decoder class. Let's split this PR into multiple chunks. One of the future PRs should be to make a decoding task graph that contains D rather than a decoder containing D. Once that is done, that would affect all D parts of this PR.

Per our discussion, the other pieces should probably be moved into individual PRs and merged independently of the aforementioned decoding task graph object.

Originally posted by @bmhowe23 in #765 (comment)

[vishnunathasuresh]

Hi, I’m a quantum enthusiast interested in fault-tolerant quantum software and would like to work on this. My rough plan would be to separate the D matrix from the decoder into an explicit decoding-task-graph object, migrate ownership and lifetime through the affected APIs incrementally, and add C++/Python regression tests for reuse, copy/move, cleanup, and decoder execution. I’d keep the intermediate PRs small so behavior can be validated at each boundary. Would you be open to assigning this to me?

[melody-ren]

Hello @vishnunathasuresh, thank you for your interest! We appreciate the thought you put into the potential solution.
However, CUDA-Q QEC has both public and closed-source components, and the D ownership issue affects code and decoder integrations in both repositories. We therefore need to design and validate the public and private changes together.
For that reason, this is not a suitable issue for an external contribution at the moment, and we plan to keep it with the maintainers. Thank you again for offering to help.

[vishnunathasuresh]

Thanks for the clarification, @melody-ren — that makes sense given the changes need to stay coordinated across the public and closed-source components.

I’ll leave this one with the maintainers. I’d still be interested in contributing to CUDA-Q QEC, so if there are issues that are fully contained within the public repository and suitable for external contributors, I’d be happy to take a look.

Thanks again!