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/app/cmd_result.rs

@@ -723,7 +723,7 @@ impl OpenCADStudio {
                             },
                         );
                     }
-                    // Step 3: rotate in XZ plane by angle_rad
+                    // Step 3: rotate in the XY plane by angle_rad
                     if angle_rad.abs() > 1e-4 {
                         self.tabs[i].scene.transform_entities(
                             &handles,
@@ -989,16 +989,16 @@ impl OpenCADStudio {
                 let mut count = 0usize;
 
                 // Helper: is DXF point (x, y) inside the world-space window?
-                // World XZ = DXF XY.
+                // Drawing plane is world XY (= DXF XY).
                 let in_win = |x: f64, y: f64| -> bool {
                     let wx = x as f32;
                     let wy = y as f32;
-                    wx >= win_min.x && wx <= win_max.x && wy >= win_min.z && wy <= win_max.z
+                    wx >= win_min.x && wx <= win_max.x && wy >= win_min.y && wy <= win_max.y
                 };
 
                 let dx = delta.x as f64;
-                let dy = delta.z as f64; // world Z = DXF Y
-                let dz = delta.y as f64;
+                let dy = delta.y as f64; // drawing plane is world XY
+                let dz = delta.z as f64;
 
                 for handle in &handles {
                     let Some(entity) = self.tabs[i].scene.document.get_entity_mut(*handle) else {

src/app/mod.rs

@@ -252,7 +252,7 @@ pub(super) struct OpenCADStudio {
     update_notice_body: Option<String>,
     /// In-memory clipboard: cloned entities waiting to be pasted.
     clipboard: Vec<acadrust::EntityType>,
-    /// Centroid of the clipboard entities (XZ plane, Y-up).
+    /// Centroid of the clipboard entities (world XY plane).
     clipboard_centroid: glam::Vec3,
     /// True while the Shift key is held — drives subtractive pick (Shift+click
     /// removes the picked entity from the selection). Tracked from keyboard