M8.10: Perspective grid view options: Lock Grid, Lock Station Point, Snap to Grid, Show Rulers

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>

docs/control-protocol.md

@@ -511,3 +511,7 @@ A selected blend shows its spine. With Direct Selection (`directSelection`) a `u
 moves it (`object.blend.spine.moveAnchor`, one undo step) and, once a point is clicked, from one of its handles
 reshapes the curve; Direct and Group Selection pick a blend's key objects. With the Pen tool a click on a selected
 blend's spine adds a point (on a point no key sits on, deletes it).
+
+Perspective grid view options: View → Perspective Grid lists Show/Hide Grid, Show/Hide Rulers, Snap to Grid
+(checked), Lock/Unlock Grid and Lock Station Point (checked); `ui.menu.list` reports the current labels. They run
+`perspective.grid.show`, `rulers`, `snap`, `lock` and `lockStation`.