fix(commands): finish the world-XY coordinate migration (#54)

The drawing plane is world XY, but break, join, rectangular array, the
stretch/window move, the spline preview wire and several doc comments
still used the stale "Y-up XZ" mapping (world Z = DXF Y). That made
those commands operate against a constant-elevation axis: arc/circle/
line/spline break and join picked the wrong points, rectangular array
stepped rows along the Z (elevation) axis, window-stretch tested the
wrong window bound, and the spline wire drew in the wrong plane.

Map every drawing-plane coordinate through .y (elevation on .z),
matching the entity builders, the inquiry commands and the central
transform. Stale Y-up comments updated to avoid future confusion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/modules/home/modify/array.rs

@@ -760,11 +760,12 @@ impl Array3DCommand {
                     if l == 0 && r == 0 && c == 0 {
                         continue;
                     }
-                    // World: X=col dir, Z=row dir (DXF Y), Y=level (up)
+                    // Drawing plane is world XY: X = col dir, Y = row dir,
+                    // Z = level (elevation).
                     t.push(EntityTransform::Translate(Vec3::new(
                         col_sp * c as f32,
-                        lvl_sp * l as f32,
                         row_sp * r as f32,
+                        lvl_sp * l as f32,
                     )));
                 }
             }