Serious business Bear MK2, MK2.5, MK3 y-axis replacement

Background
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I wanted a stiffer, more-awesome Y axis for my Bear printer. So I replaced everything on the axis except for the motor mount. That's heavy-handed for a lot of folks, so I expect *most people will go half-way* on this project.

### Half-way
You replace the idler, belt holder, and motor holder, but in return you get:
- Moves the belt tension path to be parallel to the carriage.
- Provides a tensioner.
- Provides an end-stop that doesn't involve bashing the bearings in