feat(grip): value-prompt actions Lengthen/Radius/Arc Length

Lengthen / Radius / Arc Length need a number after the pick. Stash
`grip_pending` on the picked action, push prompt to command line,
route next `CommandSubmit` parse into `apply_grip_menu_value`.
Escape cancels.

- `Grippable::grip_menu_value_prompt` returns label when action
  wants value; `apply_grip_menu_value` runs the edit. Dispatched
  via `EntityTypeOps`.
- Line: Lengthen extends either endpoint along line direction.
- Arc: Radius sets radius; Arc Length redrives end angle from
  `value / radius`; Lengthen extends start / end angle the same
  way.

src/entities/arc.rs

@@ -253,8 +253,55 @@ impl crate::entities::traits::Grippable for Arc {
         _grip_id: usize,
         _action: crate::scene::object::GripMenuAction,
     ) {
-        // Radius / Arc Length / Lengthen all need a follow-up prompt
-        // (numeric distance / length / radius) — Phase 3.
+        // Radius / Arc Length / Lengthen all need a follow-up prompt;
+        // the actual edit happens in `apply_grip_menu_value`.
+    }
+
+    fn grip_menu_value_prompt(
+        &self,
+        _grip_id: usize,
+        action: crate::scene::object::GripMenuAction,
+    ) -> Option<&'static str> {
+        use crate::scene::object::GripMenuAction as A;
+        match action {
+            A::Radius => Some("New radius"),
+            A::ArcLength => Some("New arc length"),
+            A::Lengthen => Some("Distance"),
+            _ => None,
+        }
+    }
+
+    fn apply_grip_menu_value(
+        &mut self,
+        grip_id: usize,
+        action: crate::scene::object::GripMenuAction,
+        value: f64,
+    ) {
+        use crate::scene::object::GripMenuAction as A;
+        match action {
+            A::Radius if value > 0.0 => self.radius = value,
+            A::ArcLength if value > 0.0 && self.radius > 1e-9 => {
+                // Hold start_angle, derive new end_angle from arc length
+                // = r * Δθ.
+                let new_span = value / self.radius;
+                self.end_angle = self.start_angle + new_span;
+            }
+            A::Lengthen => {
+                // Extend either end by `value` arc-length units along
+                // the arc. Positive `value` lengthens; negative
+                // shortens. Grip 1 = start endpoint, grip 2 = end endpoint.
+                if self.radius < 1e-9 {
+                    return;
+                }
+                let dtheta = value / self.radius;
+                match grip_id {
+                    1 => self.start_angle -= dtheta,
+                    2 => self.end_angle += dtheta,
+                    _ => {}
+                }
+            }
+            _ => {}
+        }
     }
 }
 

src/entities/line.rs

@@ -182,7 +182,55 @@ impl crate::entities::traits::Grippable for Line {
         _grip_id: usize,
         _action: crate::scene::object::GripMenuAction,
     ) {
-        // Lengthen requires a follow-up distance prompt — Phase 3.
+        // Lengthen needs a follow-up distance — handled by
+        // `apply_grip_menu_value`.
+    }
+
+    fn grip_menu_value_prompt(
+        &self,
+        _grip_id: usize,
+        action: crate::scene::object::GripMenuAction,
+    ) -> Option<&'static str> {
+        use crate::scene::object::GripMenuAction as A;
+        match action {
+            A::Lengthen => Some("Distance"),
+            _ => None,
+        }
+    }
+
+    fn apply_grip_menu_value(
+        &mut self,
+        grip_id: usize,
+        action: crate::scene::object::GripMenuAction,
+        value: f64,
+    ) {
+        use crate::scene::object::GripMenuAction as A;
+        if !matches!(action, A::Lengthen) {
+            return;
+        }
+        let dx = self.end.x - self.start.x;
+        let dy = self.end.y - self.start.y;
+        let dz = self.end.z - self.start.z;
+        let len = (dx * dx + dy * dy + dz * dz).sqrt();
+        if len < 1e-12 {
+            return;
+        }
+        let (ux, uy, uz) = (dx / len, dy / len, dz / len);
+        match grip_id {
+            0 => {
+                // Move start endpoint backward along the line by `value`
+                // (positive = lengthen; negative = shorten).
+                self.start.x -= ux * value;
+                self.start.y -= uy * value;
+                self.start.z -= uz * value;
+            }
+            1 => {
+                self.end.x += ux * value;
+                self.end.y += uy * value;
+                self.end.z += uz * value;
+            }
+            _ => {}
+        }
     }
 }
 

src/entities/traits.rs

@@ -35,6 +35,28 @@ pub trait Grippable {
     /// Entities override this to mutate themselves when the user picks
     /// Add Vertex / Remove Vertex / Convert / Reverse / …
     fn apply_grip_menu(&mut self, _grip_id: usize, _action: GripMenuAction) {}
+    /// Same as `apply_grip_menu` but for actions that need a numeric
+    /// follow-up (Lengthen / Radius / Arc Length / Rotate Text / …).
+    /// The app prompts the user for a value on the command line and
+    /// calls this with the parsed `f64`.
+    fn apply_grip_menu_value(
+        &mut self,
+        _grip_id: usize,
+        _action: GripMenuAction,
+        _value: f64,
+    ) {}
+    /// If the popup action this entity defines for `grip_id` needs a
+    /// numeric prompt, return the prompt string and the action so the
+    /// app can stash a pending-value state and route the next typed
+    /// number into `apply_grip_menu_value`. `None` for one-shot
+    /// actions handled by `apply_grip_menu`.
+    fn grip_menu_value_prompt(
+        &self,
+        _grip_id: usize,
+        _action: GripMenuAction,
+    ) -> Option<&'static str> {
+        None
+    }
 }
 
 pub trait PropertyEditable {
@@ -78,6 +100,17 @@ pub trait EntityTypeOps {
     fn apply_geom_prop(&mut self, field: &str, value: &str);
     fn apply_grip(&mut self, grip_id: usize, apply: GripApply);
     fn apply_grip_menu(&mut self, grip_id: usize, action: GripMenuAction);
+    fn grip_menu_value_prompt(
+        &self,
+        grip_id: usize,
+        action: GripMenuAction,
+    ) -> Option<&'static str>;
+    fn apply_grip_menu_value(
+        &mut self,
+        grip_id: usize,
+        action: GripMenuAction,
+        value: f64,
+    );
     fn apply_transform(&mut self, t: &EntityTransform);
     fn mass_props(&self) -> Option<MassProps>;
     fn text_content(&self) -> Option<String>;
@@ -287,6 +320,47 @@ impl EntityTypeOps for EntityType {
         )
     }
 
+    fn grip_menu_value_prompt(
+        &self,
+        grip_id: usize,
+        action: GripMenuAction,
+    ) -> Option<&'static str> {
+        dispatch!(self,
+            |e| Grippable::grip_menu_value_prompt(e, grip_id, action),
+            [
+                Line, Circle, Arc, Ellipse, LwPolyline, Polyline, Polyline2D,
+                Polyline3D, Ray, XLine, RasterImage, Wipeout,
+                AttributeDefinition, AttributeEntity, MLine, Tolerance,
+                Solid, Solid3D, Region, Body, Face3D, PolygonMesh,
+                PolyfaceMesh, Mesh, Table, Point, Spline, Text, MText,
+                Viewport, Insert, Leader, MultiLeader, Dimension, Hatch,
+                Underlay, Shape, Ole2Frame,
+            ],
+            _ => None,
+        )
+    }
+
+    fn apply_grip_menu_value(
+        &mut self,
+        grip_id: usize,
+        action: GripMenuAction,
+        value: f64,
+    ) {
+        dispatch!(self,
+            |e| Grippable::apply_grip_menu_value(e, grip_id, action, value),
+            [
+                Line, Circle, Arc, Ellipse, LwPolyline, Polyline, Polyline2D,
+                Polyline3D, Ray, XLine, RasterImage, Wipeout,
+                AttributeDefinition, AttributeEntity, MLine, Tolerance,
+                Solid, Solid3D, Region, Body, Face3D, PolygonMesh,
+                PolyfaceMesh, Mesh, Table, Point, Spline, Text, MText,
+                Viewport, Insert, Leader, MultiLeader, Dimension, Hatch,
+                Underlay, Shape, Ole2Frame,
+            ],
+            _ => {},
+        )
+    }
+
     fn apply_transform(&mut self, t: &EntityTransform) {
         dispatch!(self,
             |e| Transformable::apply_transform(e, t),