M8.13: Perspective planes move along their normals with widgets, alone, with or copying their objects

The grid keeps per-plane offsets (leftOffset, rightOffset, groundOffset). Each plane has a widget
(a diamond in its colour) that the Perspective Grid and Perspective Selection tools drag: plain
moves the plane, Shift moves the objects on it too, Alt copies them (perspective.plane.move).
Double-clicking a widget, or ui.perspectivePlane, opens the plane's options dialog (location; Do
Not Move / Move All Objects / Copy All Objects). Object › Perspective › Move Plane to Match Object
(perspective.plane.matchObject). New objects attach where the plane is. Grid edits leave
attached art in place as the reference app does; perspective.grid.set {reproject: true} moves it
with the grid.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/tests/command_sweep.rs

@@ -230,6 +230,12 @@ fn structured_junk() {
         ("perspective.transform", json!({"matrix": [1, 0, 0, 1, 0, 0], "depth": -1e9, "plane": "left", "copy": true})),
         ("perspective.transform", json!({"matrix": [1, 0, 0, 1, 1e300, 0], "plane": "left"})),
         ("perspective.nudge", json!({"dx": 1e308, "dy": -1e308, "big": true, "copy": true})),
+        ("perspective.plane.move", json!({"plane": "ground", "offset": 1e308, "objects": "copy"})),
+        ("perspective.plane.move", json!({"plane": "left", "by": -1e7, "objects": "move"})),
+        ("perspective.plane.move", json!({"plane": "none", "offset": 0})),
+        ("perspective.plane.matchObject", json!({"id": u64::MAX})),
+        ("perspective.grid.set", json!({"leftOffset": -1e12, "reproject": true})),
+        ("perspective.grid.set", json!({"vpRight": -1e6, "reproject": true})),
         ("object.setProps", json!({"opacity": -1, "blend": 5})),
         ("effect.apply", json!({"effect": "distort.roughen", "params": {"size": 100, "detail": 100, "relative": false}})),
         ("effect.apply", json!({"effect": "distort.zigZag", "params": {"size": 1e308, "ridges": 100}})),