[BUG] MSM bug fixes

[chrisholder]

Reference Issues/PRs

What does this implement/fix? Explain your changes.

Summary of Fixes to MSM Implementation

This PR addresses a bug in dependent MSM implementations and cleans up some of the code.

1. Dependent MSM “in-between” cost threshold (more significant)

The cost function for dependent MSM was incorrectly using diameter / 2 as the threshold for determining whether a point lies inside the hypersphere defined by two others.
This threshold is correct when using Euclidean distance, but our implementation uses squared Euclidean distance.

When using squared distances, the correct condition is:

$|mid - x|^2 \le \frac{|y - z|^2}{4}$

Therefore, the denominator must be 4, not 2.
The implementation has now been updated to divide by 4, ensuring correctness relative to the published definition of dependent MSM.

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