fix: Spline snap points and TextStyle backward/upside-down rendering

Spline entities now expose fit_points (or control_points when no fit
points are defined) as key_vertices, enabling Endpoint and Midpoint
snap to reach spline construction points.

TextStyle is_backward flag is applied by negating the width_factor,
producing horizontally mirrored text. is_upside_down rotates the text
180°. Both tessellate_text_ex and text_local_bounds now handle negative
width_factor without clamping it to zero.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/entities/spline.rs

@@ -37,11 +37,24 @@ fn to_truck(spl: &Spline) -> TruckEntity {
     let v_start = builder::vertex(p_start);
     let v_end = builder::vertex(p_end);
     let edge = Edge::new(&v_start, &v_end, Curve::BSplineCurve(bspline));
+
+    // Expose fit_points (preferred) or control_points as key_vertices so that
+    // Endpoint/Midpoint snap can reach spline construction points.
+    let snap_source = if !spl.fit_points.is_empty() {
+        &spl.fit_points
+    } else {
+        &spl.control_points
+    };
+    let key_vertices: Vec<[f32; 3]> = snap_source
+        .iter()
+        .map(|p| [p.x as f32, p.y as f32, p.z as f32])
+        .collect();
+
     TruckEntity {
         object: TruckObject::Curve(edge),
         snap_pts: vec![],
         tangent_geoms: vec![],
-        key_vertices: vec![],
+        key_vertices,
     }
 }