feat(grip): popup menus for polylines, spline, mline, hatch, leaders, text

Vocabulary added: AddFitPoint, RemoveFitPoint, Refit, RefineVertices,
TangentDirection, MoveWithText, StackText, UnstackText.

Polyline/Polyline2D/Polyline3D: vertex Add/Remove. MLine: same.
Spline: CV Add/Remove plus Refine Vertices (chord-midpoint insertion,
weights kept in sync, knots cleared so to_truck rebuilds uniform vec).
Hatch: centroid Origin/Angle/Scale (Angle and Scale prompt for value).
Leader: vertex Add/Remove (arrow + centroid stay Stretch-only).
MultiLeader: vertex Add/Remove Leader; MText location Move-with-Leader /
Move-Independent (menu live, structural edits stubbed). Text/MText:
Rotate prompts for degrees and sets rotation.

Spline, Leader, MultiLeader switched from impl_entity_basics! to manual
trait impls so grip_menu can be overridden.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/entities/mline.rs

@@ -152,6 +152,46 @@ impl Grippable for MLine {
             }
         }
     }
+
+    fn grip_menu(
+        &self,
+        _grip_id: usize,
+    ) -> Vec<crate::scene::object::GripMenuItem> {
+        use crate::scene::object::{GripMenuAction, GripMenuItem};
+        vec![
+            GripMenuItem { label: "Stretch", action: GripMenuAction::Stretch },
+            GripMenuItem { label: "Add Vertex", action: GripMenuAction::AddVertex },
+            GripMenuItem { label: "Remove Vertex", action: GripMenuAction::RemoveVertex },
+        ]
+    }
+
+    fn apply_grip_menu(
+        &mut self,
+        grip_id: usize,
+        action: crate::scene::object::GripMenuAction,
+    ) {
+        use crate::scene::object::GripMenuAction as A;
+        let n = self.vertices.len();
+        match action {
+            A::AddVertex if grip_id < n => {
+                let i1 = (grip_id + 1).min(n - 1);
+                if i1 == grip_id {
+                    return;
+                }
+                let v0 = &self.vertices[grip_id];
+                let v1 = &self.vertices[i1];
+                let mut new_v = v0.clone();
+                new_v.position.x = (v0.position.x + v1.position.x) * 0.5;
+                new_v.position.y = (v0.position.y + v1.position.y) * 0.5;
+                new_v.position.z = (v0.position.z + v1.position.z) * 0.5;
+                self.vertices.insert(i1, new_v);
+            }
+            A::RemoveVertex if grip_id < n && n > 2 => {
+                self.vertices.remove(grip_id);
+            }
+            _ => {}
+        }
+    }
 }
 
 impl PropertyEditable for MLine {