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/scene/grip.rs

@@ -36,7 +36,7 @@ pub fn grips_to_screen(
     grips: &[GripDef],
     view_proj: Mat4,
     bounds: Rectangle,
-) -> Vec<(usize, Point, bool, GripShape)> {
+) -> Vec<(usize, Point, bool, GripShape, Option<[f32; 2]>)> {
     grips
         .iter()
         .map(|g| {
@@ -45,7 +45,7 @@ pub fn grips_to_screen(
                 bounds.x + (ndc.x + 1.0) * 0.5 * bounds.width,
                 bounds.y + (1.0 - ndc.y) * 0.5 * bounds.height,
             );
-            (g.id, screen, g.is_midpoint, g.shape)
+            (g.id, screen, g.is_midpoint, g.shape, g.dir)
         })
         .collect()
 }
@@ -59,7 +59,7 @@ pub fn grips_to_screen_paper(
     half_w: f32,
     half_h: f32,
     bounds: Rectangle,
-) -> Vec<(usize, Point, bool, GripShape)> {
+) -> Vec<(usize, Point, bool, GripShape, Option<[f32; 2]>)> {
     grips
         .iter()
         .map(|g| {
@@ -67,7 +67,7 @@ pub fn grips_to_screen_paper(
                 (g.world.x - tx + half_w) / (2.0 * half_w) * bounds.width,
                 (ty + half_h - g.world.y) / (2.0 * half_h) * bounds.height,
             );
-            (g.id, screen, g.is_midpoint, g.shape)
+            (g.id, screen, g.is_midpoint, g.shape, g.dir)
         })
         .collect()
 }

src/scene/object.rs

@@ -51,23 +51,23 @@ pub struct PropSection {
 
 // ── Grip types ─────────────────────────────────────────────────────────────
 
-/// Visual marker shape for a grip point. The complete vocabulary used
-/// across entity types:
-/// * `Square` — vertex / endpoint that moves a single point.
-/// * `Rectangle` — direction-aware mid-segment stretch (polyline
-///   straight segments, dimension extension lines).
-/// * `Diamond` — midpoint of a curve / centre of a closed shape (drags
-///   the whole shape or stretches the midpoint).
-/// * `Triangle` — directional control (rotate / add vertex / continue).
-/// * `Circle` — parametric control (radius / dimension value).
+/// Visual marker shape for a grip point. The complete vocabulary that
+/// matches the standard CAD grip conventions:
+/// * `Square` — endpoint / vertex / centre. Drag → moves a single
+///   point or translates the entity.
+/// * `Rectangle` — direction-aware mid-segment stretch handle.
+///   Drawn as a small box rotated along `dir` (the in-plane segment
+///   direction in world XY). Used for polyline / wipeout / image /
+///   dimension segment midpoints.
+/// * `Triangle` — directional indicator (Phase 2: dynamic-block
+///   parameters, dimension reverse-arrow flips, multi-functional
+///   hover popups).
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 #[allow(dead_code)]
 pub enum GripShape {
     Square,
     Rectangle,
-    Diamond,
     Triangle,
-    Circle,
 }
 
 /// Describes one grip point for an entity.
@@ -77,11 +77,15 @@ pub struct GripDef {
     pub id: usize,
     /// World-space position of the grip.
     pub world: Vec3,
-    /// `true` → midpoint grip (diamond, translates whole object).
-    /// `false` → endpoint grip (square, moves a single vertex).
+    /// `true` → midpoint / centre grip (drags the whole shape).
+    /// `false` → endpoint / vertex grip (moves a single point).
     pub is_midpoint: bool,
     /// Visual marker shape for the grip.
     pub shape: GripShape,
+    /// World-XY direction vector used to orient a `Rectangle` grip
+    /// along its segment. `None` for shapes that don't need rotation
+    /// (Square, Triangle in non-directional contexts).
+    pub dir: Option<[f32; 2]>,
 }
 
 /// How to apply a grip drag result.