• Add support for snap to unit grid to Line, Arc, and any segment-based tools
• Make it a setting that users can toggle

We will likely need to conditionally round the current values when moving the mouse around based on the units of the current grid scale (or the current default units but that could get broken real quick if you're zoomed into mm and your default unit is m?).

In order to surface it to settings we just need to make that value come from the settingsMachine's context, and add a machine event and corresponding palette command to toggle it.