refactor(grip): square-only marker set + oriented stretch handles

Phase 1 of bringing the grip vocabulary in line with the standard
CAD convention: drop the per-position-type shape encoding (Diamond
for translate, Circle for parameter) and use a plain square for
every endpoint / vertex / centre grip. The behavioural midpoint vs
vertex distinction stays — it lives on `GripDef::is_midpoint` and
drives `Translate` vs `Absolute` in the grip-edit dispatcher — only
the marker shape collapses.

- `GripShape` keeps `Square`, `Rectangle`, `Triangle`. Diamond /
  Circle are gone.
- `GripDef` gains `dir: Option<[f32; 2]>` — a world-XY direction
  vector consumed by `Rectangle` to orient the box along its
  segment.
- `entities::common` exposes `square_grip`, `center_grip` (same
  square marker, flagged as translate), `rectangle_grip(id, world,
  dir)` for oriented mid-segment handles, and a kept-for-Phase-2
  `triangle_grip`.
- `overlay::GripMarker` / `grips_to_screen[_paper]` thread `dir`
  through to the canvas painter; the Rectangle path rotates the
  box around the grip centre using the world-XY direction (flipping
  the sin for screen-Y).

Every existing `diamond_grip(id, w)` call becomes `center_grip(id,
w)` — the visual change is square-replacing-diamond at line
midpoints / arc midpoints / circle centres / ellipse centres /
viewport / hatch loop centroids / dimension grips / image corners /
multileader / ray / underlay / ole2frame / solid3d.

`LwPolyline` grows a stretch handle on every segment (straight or
arc), drawn as the new oriented rectangle. The grip-edit path
translates both segment endpoints for straight segments and
adjusts the arc bulge from the new midpoint for arc segments.

src/entities/underlay.rs

@@ -8,7 +8,7 @@ use acadrust::entities::{Underlay, UnderlayDisplayFlags};
 use glam::Vec3;
 
 use crate::command::EntityTransform;
-use crate::entities::common::{diamond_grip, edit_prop as edit, ro_prop as ro, square_grip};
+use crate::entities::common::{center_grip, edit_prop as edit, ro_prop as ro, square_grip};
 use crate::entities::traits::{Grippable, PropertyEditable, Transformable, TruckConvertible};
 use crate::scene::acad_to_truck::{TruckEntity, TruckObject};
 use crate::scene::object::{GripApply, GripDef, PropSection};
@@ -87,7 +87,7 @@ impl Grippable for Underlay {
         if !self.clip_boundary_vertices.is_empty() {
             let world_verts = self.world_clip_boundary();
             for (i, v) in world_verts.iter().enumerate() {
-                grips.push(diamond_grip(
+                grips.push(center_grip(
                     i + 1,
                     Vec3::new(v.x as f32, v.y as f32, v.z as f32),
                 ));