Smaller 2U Bracket

The `2U Mini 4x LabStack Module x Bracket` variants are too large to print on a Prusa MK4S print bed. I tried the split version however I found that:

1) It's very unstable/flimsy when assembled and;
2) It doesn't come with a version for the `M3x3x4.6mm` heatset insert which I need.

I decided to 'remix' the `2U Mini 4x LabStack Module M3x3x4.6mm Heatset Bracket` version, shortening it just enough that it can be printed on the MK3S print bed while still fitting in the rack cage nuts.

You can find the source design in onshape here:

https://cad.onshape.com/documents/ff7fd8254db6bb3b4eb500a6/w/bf2aceda4fc5bfa4d316356b/e/f253633dd807d88f137cbb58

Or you can reference the STL file I attached here:

[2U Bracket - 2U Bracket.stl.zip](https://github.com/user-attachments/files/22280181/2U.Bracket.-.2U.Bracket.stl.zip)

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As I designed this from scratch only using the files from here as reference, it is not visually identical in every way. It is identical for all important dimensions.

The one relevant dimension I needed to change is the rack attachment holes which I needed to shorten very slightly. With my changes this _just_ fits on my [3D printed Mod 10 Rack](https://www.printables.com/model/1225275-modular-10-server-rack-mod10). I don't know how portable this is to other racks, but I assume it should work.

I'm putting all this here so that others can use this if they have the same problems as me, and also to let you know of the success I had with this modification. Please feel free to use this as reference if you want to make similar alterations and include short variations of the brackets in this repo.

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Start with the linked Onshape source design and compare it with the attached STL for the 2U Mini 4x LabStack Module M3x3x4.6mm Heatset Bracket. Check the shortened rack attachment holes, MK3S/MK4S print-bed fit, stability, and cage-nut compatibility; done means a short bracket variation is available for others to use.

Written by the indexing model from the issue text.