Guide lines

Drag from a ruler to create a horizontal or vertical guide line. (Drag it back into the ruler to delete it.) This is positioned in document space. *View* > *Guides* should go next to the existing *View* > *Rulers* menu to toggle their visibility. Snapping works with them like they're a line (just without endpoints, since guide lines are infinitely long). Like everything else that you can drag in Graphite, Esc and right-click cancel a drag, putting it back where it began (or deleting it if this was just brought into existence by dragging out of a ruler).

Start by locating the existing ruler implementation and the View > Rulers menu, then trace how dragging and snapping are handled in document space. Check the relevant view and interaction tests, if present. Done means guides can be created and deleted from rulers, toggled through View > Guides, participate in snapping, and cancel correctly with Esc or right-click.

Written by the indexing model from the issue text.