Path tool dragging, then pressing Shift to constrain horiz./vert., should begin from a snapping point

At the beginning of this video, I add Shift to the drag and it correctly performs the horizontal/vertical snapping from its original drag location. But in the second half of the video, I demonstrate a use case where this feature is desired: when the point is dragged such that it snaps over another point, then Shift is pressed at that time, we would like that snapped point to become the source of the horizontal/vertical constraint. Releasing Shift and pressing it again while not snapped should retain its current behavior of constraining relative to the point where the drag began. As shown in the video, this feature would make it very easy to keep the top point aligned to the same X coordinate as the bottom point.

https://github.com/user-attachments/assets/e59f5531-8b8a-4a18-a5ab-520fef992aa8

Start by reproducing the attached video and locating the Path tool's dragging and snapping entry points. Read how Shift constraints choose their reference point; done means a Shift press while snapped uses that snapped point, while later presses away from a snap still use the original drag location.

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