feat(grip): multi-functional hover popup menu

Phase 2 of the grip overhaul: when the cursor dwells on a selected
entity's grip the overlay opens a popup menu with entity-specific
options (Stretch / Add Vertex / Remove Vertex / Convert to
Arc-Line / …). Picking an item dispatches `apply_grip_menu`; the
default `Stretch` entry is a no-op that lets the user click the
grip to start the normal drag.

- `GripMenuItem` + `GripMenuAction` (full vocabulary covering
  stretch, add/remove vertex, convert to arc/line, reverse arrows,
  text-position variants, hatch parameters, tangent direction).
- `Grippable` trait gains `grip_menu` + `apply_grip_menu` with
  trait-default `Stretch`-only impls so every existing entity gets
  the popup for free.
- `EntityTypeOps` dispatches both methods across the same entity
  list as the existing grip / property machinery.
- `OpenCADStudio` carries `grip_hover` (dwell tracker) +
  `grip_popup` (open menu). `update_grip_hover` runs from
  `ViewportMove`: hit-tests grips, refreshes the dwell timer, and
  opens the popup after 600 ms. Cursor drift past 80 px or a grip
  click dismisses it.
- `view.rs` paints the popup as an absolute-positioned column of
  buttons at the grip's screen anchor; each button publishes
  `Message::GripMenuPick(idx)`.
- `LwPolyline` overrides `Grippable` manually (skips the standard
  macro) so its vertex grip exposes Add/Remove Vertex and its
  segment-midpoint grip exposes Add Vertex + Convert to Arc/Line.
  `apply_grip_menu` performs the edits: vertex insertion at the
  chord midpoint inheriting the previous bulge, vertex removal,
  and bulge toggle for the arc/line conversion.

src/app/view.rs

@@ -684,6 +684,69 @@ impl OpenCADStudio {
             }
         }
 
+        // Multi-functional grip popup (Phase 2). Floating column of
+        // menu items at the grip's screen position, with the selected
+        // item highlighted. Each item dispatches `GripMenuPick`.
+        if let Some(popup) = self.grip_popup.as_ref() {
+            if !tab.is_start {
+                let anchor_x = (popup.anchor.x + 12.0).max(0.0);
+                let anchor_y = (popup.anchor.y + 12.0).max(0.0);
+                let mut col = column![].spacing(0);
+                for (idx, item) in popup.items.iter().enumerate() {
+                    let is_sel = idx == popup.selected;
+                    let label = item.label;
+                    let btn = button(text(label).size(12).color(Color::WHITE))
+                        .on_press(Message::GripMenuPick(idx))
+                        .padding([4, 10])
+                        .style(move |_: &Theme, status| iced::widget::button::Style {
+                            background: Some(Background::Color(match (is_sel, status) {
+                                (true, _) => Color {
+                                    r: 0.20,
+                                    g: 0.45,
+                                    b: 0.95,
+                                    a: 0.95,
+                                },
+                                (_, iced::widget::button::Status::Hovered) => Color {
+                                    r: 0.20,
+                                    g: 0.20,
+                                    b: 0.20,
+                                    a: 0.95,
+                                },
+                                _ => Color {
+                                    r: 0.10,
+                                    g: 0.10,
+                                    b: 0.10,
+                                    a: 0.92,
+                                },
+                            })),
+                            border: Border {
+                                color: Color {
+                                    r: 0.35,
+                                    g: 0.35,
+                                    b: 0.35,
+                                    a: 1.0,
+                                },
+                                width: 1.0,
+                                radius: 0.0.into(),
+                            },
+                            text_color: Color::WHITE,
+                            ..Default::default()
+                        });
+                    col = col.push(iced::widget::opaque(btn));
+                }
+                let popup_layer = iced::widget::column![
+                    Space::new().height(iced::Length::Fixed(anchor_y)),
+                    iced::widget::row![
+                        Space::new().width(iced::Length::Fixed(anchor_x)),
+                        col,
+                    ],
+                ]
+                .width(Fill)
+                .height(Fill);
+                viewport_stack = viewport_stack.push(popup_layer);
+            }
+        }
+
         // Properties / layers panels carry no useful state on the Start tab.
         // Replace the properties panel with a Recent Documents list there.
         let properties_el: Element<'_, Message> = if tab.is_start {