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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從連結的 Onshape 原始設計開始，並將其與 2U Mini 4x LabStack Module M3x3x4.6mm Heatset Bracket 的附件 STL 進行比較。檢查縮短後的機架安裝孔、與 MK3S/MK4S 列印床的適配性、穩定性以及與籠式螺帽的相容性；完成的標準是提供一個可供他人使用的短款 bracket 變體。

由索引模型根據 Issue 內容生成。