fix: Cholesky::new returns None non-positive-definite complex matrices

[DiogoRibeiro7]

For complex types, always succeeds (every complex number has a square root), so non-PD matrices were silently producing garbage results. Fix by calling on the real part of each diagonal pivot instead — matching the behaviour already correct for .

Fixes #1536

[alexhroom]

first just to note this has already been fixed in #1574 - but if you're able to answer review comments faster than that I'll probably merge this instead.

Secondly before I review, there seems to be some commits that aren't relevant to your PR here - do you mind tidying them up?

[DiogoRibeiro7]

first just to note this has already been fixed in #1574 - but if you're able to answer review comments faster than that I'll probably merge this instead.

Secondly before I review, there seems to be some commits that aren't relevant to your PR here - do you mind tidying them up?

Thanks for the suggestion! I went a slightly different route — instead of explicit checks, the closure now just does v.real().try_sqrt().map(T::from_real). This way try_sqrt() on the real part naturally returns None for negative/zero values, and the imaginary part is intentionally ignored to tolerate small floating-point residuals (for Hermitian matrices the diagonal stays real throughout the algorithm). I also cleaned up the unrelated commits.