build(deps): bump acadrust to 486c0a9; handle the Helix entity

Pull in this session's acadrust work — the HELIX / ACAD_TABLE read+write,
underlay reader/writer/definition, spline tolerances & tangents, etc. The new
EntityType::Helix variant breaks three exhaustive matches; add its arm to
entity_type_name, the UI-name / DXF-name maps and entity_type_key.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/helpers.rs

@@ -313,6 +313,7 @@ pub(super) fn entity_type_key(entity: &acadrust::EntityType) -> String {
         Arc(_) => "arc",
         Ellipse(_) => "ellipse",
         Spline(_) => "spline",
+        Helix(_) => "helix",
         LwPolyline(_) | Polyline(_) => "pline",
         Polyline2D(_) => "pline2d",
         Polyline3D(_) => "pline3d",