fix(extend): use preselected objects as boundary edges

TRIM already takes objects selected before the command as cutting
edges; EXTEND ignored them and extended to the nearest object in the
path. It now takes them as boundary edges the same way.

Fixes #1732

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

src/modules/draw/modify/trim.rs

@@ -3358,21 +3358,28 @@ pub struct ExtendCommand {
 
 impl ExtendCommand {
     pub fn new(all_entities: Vec<EntityType>) -> Self {
+        Self::with_boundary_edges(all_entities, Vec::new())
+    }
+
+    /// Objects selected before the command are its boundary edges, as TRIM
+    /// takes them as cutting edges.
+    pub fn with_boundary_edges(all_entities: Vec<EntityType>, initial_edges: Vec<Handle>) -> Self {
         let all_entities: Vec<EntityType> = all_entities
             .iter()
             .map(entity_with_lwpolyline_world_xy)
             .collect();
         let entity_index = ModifyEntityIndex::build(&all_entities);
-        let geos = build_geos(&all_entities);
-        Self {
+        let mut cmd = Self {
             all_entities,
             entity_index,
-            geos,
+            geos: Vec::new(),
             mode: TrimMode::Pick,
-            edge_set: Vec::new(),
+            edge_set: initial_edges,
             implied_edges: false,
             shift: false,
-        }
+        };
+        cmd.rebuild_geos();
+        cmd
     }
 
     fn rebuild_geos(&mut self) {