Task 59743511435

refactor, key: move `CreateMuSig2{Nonce,PartialSig}` functions to `musig.{h,cpp}` module

2026-01-07 23:45:43 completed 202601-move-musig-funcs-to-module bb3e26c2b59e069f1e8f30436ea6affb52098b87

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202601-move-musig-funcs-to-module