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.
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