refactor(render): tessellate curved entities from their own curve

The point lists were built three different ways and none of them scaled
with the geometry. A full ellipse was cut into sixteen segments per half
and an elliptical arc into thirty-two, whatever its size, so anything
large read as a polygon; a circle worked out its own segment count from
the chord tolerance; an arc was handed to truck's arc-through-three-
points builder, which cancels at survey coordinates.

All three now sample the entity's curve at the render pass's chord
tolerance, so how round something looks follows how big it is and how
far in the view is zoomed. The ellipse also stops casting its local
coordinates to f32 on the way in — the vertex path splits f64 into a
double-single pair precisely so that precision survives to the shader,
and throwing it away first defeated that.

What does not change is the kind of object each entity hands over.
EXTRUDE, REVOLVE and SWEEP read their profile and path out of Contour or
Curve and have no arm for a bare point list, so an arc or an ellipse
given as Lines would quietly stop being usable as either. The arc keeps
its circle_arc edge for that reason; only the ellipse's control points
change, and they keep the same two-half-edge wire.

The ellipse now decides it is closed from its own endpoints rather than
from its stored parameters, so a file that writes an end a hair short of
a full turn still produces a wire a plane can attach to.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/entities/curve.rs

@@ -331,6 +331,22 @@ pub fn entity_curve_xy(entity: &EntityType) -> Option<Curve> {
     })
 }
 
+/// World-space wire points for a curve, cut to the render pass's current
+/// chord tolerance.
+///
+/// The density comes from how far a chord may sag from the curve, not from a
+/// segment count: an arc a metre across and one a kilometre across need
+/// different numbers of points to look equally round, and the tolerance the
+/// render pass carries is already zoom-adaptive.
+///
+/// Everything stays `f64` to the end. The narrowing belongs at the GPU
+/// boundary, where the residual is kept as the low half of a double-single
+/// pair; casting local coordinates on the way in throws away precision the
+/// shader was built to reconstruct.
+pub fn curve_points(curve: &PlanarCurve) -> Vec<[f64; 3]> {
+    curve.tessellate_within(crate::scene::convert::truck_tess::current_curve_tol())
+}
+
 /// The snap candidates an entity's curve offers, in the two channels the
 /// wire model carries them in.
 #[derive(Debug, Default, Clone)]