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/scene/cxf.rs

@@ -215,7 +215,12 @@ pub fn tessellate_text_ex(
 
     let font = get_font(font_name);
     let scale = height / 9.0;
-    let wf = width_factor.clamp(0.01, 100.0);
+    // Negative width_factor produces mirrored (backward) text.
+    let wf = if width_factor < 0.0 {
+        width_factor.clamp(-100.0, -0.01)
+    } else {
+        width_factor.clamp(0.01, 100.0)
+    };
     let ob = oblique_angle.tan();
     let (cos_r, sin_r) = (rotation.cos(), rotation.sin());