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/align.rs

@@ -151,7 +151,7 @@ impl AlignCommand {
             _ => return CmdResult::Cancel,
         };
 
-        // Build transform: move s1→d1, rotate so s2-s1 aligns with d2-d1 (in XZ plane)
+        // Build transform: move s1→d1, rotate so s2-s1 aligns with d2-d1 (in the world XY plane)
         let src_vec = s2 - s1;
         let dst_vec = d2 - d1;