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/scene/object.rs

@@ -96,3 +96,44 @@ pub enum GripApply {
     /// Translate the whole object by this delta vector.
     Translate(Vec3),
 }
+
+/// One entry in the hover-popup menu that opens when the cursor dwells
+/// on a grip. The `label` is the user-visible string; `action` is the
+/// operation the entity will perform when the item is committed.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct GripMenuItem {
+    pub label: &'static str,
+    pub action: GripMenuAction,
+}
+
+/// All operations a grip popup menu can dispatch. Entity-specific code
+/// in `apply_grip_menu` decodes these into edits. `Stretch` is the
+/// default no-op-equivalent — picking it just starts the regular
+/// stretch drag, identical to clicking the grip with no popup open.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum GripMenuAction {
+    Stretch,
+    Lengthen,
+    Radius,
+    ArcLength,
+    AddVertex,
+    RemoveVertex,
+    ConvertToArc,
+    ConvertToLine,
+    StretchVertex,
+    AddLeader,
+    RemoveLeader,
+    ReverseArrows,
+    MoveWithDimLine,
+    MoveWithLeader,
+    MoveIndependent,
+    ResetText,
+    RotateText,
+    AboveDimLine,
+    Center,
+    OriginPoint,
+    HatchAngle,
+    HatchScale,
+    HatchPattern,
+    TangentDirection,
+}