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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从链接的 Onshape 源设计开始，并将其与 2U Mini 4x LabStack Module M3x3x4.6mm Heatset Bracket 的附件 STL 进行比较。检查缩短后的机架安装孔、与 MK3S/MK4S 打印床的适配性、稳定性以及与笼式螺母的兼容性；完成的标准是提供一个可供他人使用的短款 bracket 变体。

由索引模型根据 Issue 内容生成。