feat(pline): snap onto the start point closes the polyline

A PLINE point landing back on the first vertex (endpoint snap) now
closes the polyline instead of stacking a duplicate vertex there —
including an arc closing segment, whose bulge is already computed for
the last-to-first span (#421). Line mode requires two real segments
first so a doubled-back line isn't treated as closed. Setting
Closed: Yes in the properties panel on a polyline whose last vertex
already sits on the first drops the duplicate control point for the
same reason.

Co-Authored-By: Claude Fable 5 <[email protected]>

src/entities/lwpolyline.rs

@@ -504,6 +504,18 @@ fn apply_geom_prop(pline: &mut LwPolyline, field: &str, value: &str) {
             } else {
                 value == "true"
             };
+            // Closing a polyline whose last vertex already sits on the first
+            // would stack two control points there (a polyline drawn back to
+            // its start point and then closed) — drop the duplicate so the
+            // closing segment replaces it (#421).
+            if pline.is_closed && pline.vertices.len() > 2 {
+                let (first, last) = (&pline.vertices[0], pline.vertices.last().unwrap());
+                let d2 = (first.location.x - last.location.x).powi(2)
+                    + (first.location.y - last.location.y).powi(2);
+                if d2 < 1e-12 {
+                    pline.vertices.pop();
+                }
+            }
             return;
         }
         "plinegen" => {

src/modules/draw/draw/polyline.rs

@@ -270,6 +270,20 @@ impl CadCommand for PlineCommand {
             self.bulges[last_idx] = bulge;
         }
 
+        // A point landing back on the FIRST vertex (endpoint snap) closes the
+        // polyline instead of stacking a duplicate vertex there — the segment
+        // bulge just computed above already describes the closing segment
+        // (#421). Line mode needs two real segments first so a doubled-back
+        // line isn't "closed"; an arc segment closes from two vertices.
+        if let Some(first) = self.vertices.first() {
+            let enough = self.vertices.len() >= 3
+                || (self.vertices.len() == 2 && matches!(self.mode, SegMode::Arc));
+            let d2 = (pt.x - first.x).powi(2) + (pt.y - first.y).powi(2);
+            if enough && d2 < 1e-12 {
+                return self.sync_live(true, true);
+            }
+        }
+
         self.vertices.push(pt);
         self.bulges.push(0.0);
         // Publish to the document as soon as the polyline has a segment so it