Offset of an irregular closed spline no longer fails its error certificate; offset fit nodes stay private (#1009)

Fixes #971. At the node limit the offset certificate splits the fitted curve where it meets the exact offset midpoint, so curves with different parameter speeds certify within the same 0.002 mm tolerance. Associated offset fit nodes are hidden from display, picking, box selection and snapping until Break Association.

Thanks @Karl-VonRichter.

crates/ui-egui/src/drag_snap.rs

@@ -90,7 +90,9 @@ pub fn find(sk: &Sketch, moving: &[(usize, Vec2)], exclude: &[usize], mm_px: f64
         return keep(None);
     }
     let id = |i: usize| sk.points.get(i).map(|p| p.id.clone()).unwrap_or_default();
-    let targets: Vec<(usize, Vec2)> = sk.points.iter().enumerate().filter(|(i, _)| !exclude.contains(i)).map(|(i, p)| (i, p.pos)).collect();
+    let visible = solidcraft_engine::view::SketchPointVisibility::new(sk);
+    let targets: Vec<(usize, Vec2)> =
+        sk.points.iter().enumerate().filter(|(i, p)| !exclude.contains(i) && visible.takes(p)).map(|(i, p)| (i, p.pos)).collect();
     let free_curves: Vec<usize> =
         (0..sk.curves.len()).filter(|&c| sk.curves.get(c).is_some_and(|cu| !exclude.iter().any(|&p| cu.kind.uses(p)))).collect();
     let mut best: Option<(f64, Snap)> = None;