View > Perspective Grid gains Show/Hide Rulers, Snap to Grid (on by default),
Lock/Unlock Grid and Lock Station Point, with flipping labels and checkmarks (and
Show/Hide Grid's label). Lock Grid stops the grid widgets from dragging; Lock
Station Point swings the other vanishing point around the station point; Snap to
Grid lands drawn corners and moved edges on gridlines within a quarter cell
(perspective.draw/move take snap?); Show Rulers draws a ruler up the true height
line in the grid's units. Gridlines draw in the Define Grid colours and opacity.
New commands perspective.grid.lock/lockStation/snap/rulers (view state).
Existing tests that measure exact perspective moves (two in tests_distort.rs,
the persp_session helper of tests_persp_select.rs) turn snapping off first (it is on by
default and would move their exact corners onto gridlines).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
e3b079de8c