feat: STRETCH with proper crossing-window vertex selection

Replaced the simplified MOVE-based STRETCH with a real crossing-window
implementation. Workflow: pick two crossing-window corners, then base+target
points. Only vertices/endpoints inside the window are translated:
- Line: moves start/end independently
- LwPolyline/Polyline: moves individual vertices inside window
- Arc/Circle/Ellipse: moves whole entity if center is inside
- Insert/Text/MText: moves if insertion point is inside

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/command/mod.rs

@@ -128,6 +128,16 @@ pub enum CmdResult {
     DdeditEntity { handle: Handle, new_text: String },
     /// Apply new pattern/scale/angle to an existing hatch entity.
     HatcheditApply { handle: Handle, name: String, scale: f32, angle: f32 },
+    /// Stretch entities: move only vertices/endpoints inside the crossing window.
+    StretchEntities {
+        handles: Vec<Handle>,
+        /// Min corner of the crossing window in world XZ (= DXF XY).
+        win_min: Vec3,
+        /// Max corner of the crossing window in world XZ (= DXF XY).
+        win_max: Vec3,
+        /// Translation vector to apply to vertices inside the window.
+        delta: Vec3,
+    },
 }
 
 // ── Trait ─────────────────────────────────────────────────────────────────