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/app/view.rs

@@ -299,7 +299,7 @@ impl OpenCADStudio {
                     };
                     screen_grips
                         .into_iter()
-                        .filter(|(_, screen, _, _)| {
+                        .filter(|(_, screen, _, _, _)| {
                             screen.x.is_finite()
                                 && screen.y.is_finite()
                                 && screen.x >= -bounds.width
@@ -307,7 +307,7 @@ impl OpenCADStudio {
                                 && screen.y >= -bounds.height
                                 && screen.y <= bounds.height * 2.0
                         })
-                        .map(|(grip_id, screen, _is_midpoint, shape)| {
+                        .map(|(grip_id, screen, _is_midpoint, shape, dir)| {
                             let is_hot = tab
                                 .active_grip
                                 .as_ref()
@@ -316,6 +316,7 @@ impl OpenCADStudio {
                                 pos: screen,
                                 shape,
                                 is_hot,
+                                dir,
                             }
                         })
                         .collect()