As in Photoshop, holding Shift while dragging a new guide out of a ruler
snaps it to the tick marks the ruler draws (tenths of the labelled step),
in the current ruler unit and zoom. Without Shift the guide snaps to the
document, layers, selection and grid as before.
The tick spacing draw_rulers computed inline moves to ruler_scale, shared
by the renderer and the new snap_to_tick, so the snap always matches the
ticks on screen.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>