feat(entities): render + property builder for loaded Helix entities

A helix loaded from a DWG (AcDbHelix) previously fell through to the
fallback: it did not render, had no grips, could not be moved, and showed
only a Type row. Add src/entities/helix.rs which drives all of these
through the embedded spline (render/grips/transform) and exposes the
generating parameters — base point, turns, turn height, height, base/top
radius, turn slope, twist, constrain — as the Geometry group per the
properties spec. Wire Helix into the to_truck / grips / apply_grip /
geometry_properties / apply_geom_prop / apply_transform dispatch lists.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entities/mod.rs

@@ -5,6 +5,7 @@ pub mod common;
 pub mod dimension;
 pub mod ellipse;
 pub mod hatch;
+pub mod helix;
 pub mod insert;
 pub mod leader;
 pub mod line;