Silhouette edges on curved surfaces: drawn in the surface shader where it turns away from the eye (about 1.5 px, only on the visible surface, so nothing shows through thin walls), in the viewport and live previews; snapshots draw them too. The back-face outline pass is gone

Fixes #48

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

crates/engine/src/view.rs

@@ -282,4 +282,15 @@ mod tests {
         let img = solvecraft_render::render(&sc, &cam, 200, 140);
         assert!(img.rgba.chunks(4).any(|p| p[1] > 120 && p[0] < 90 && p[2] < 110), "no green pixels");
     }
+
+    /// Curved surfaces get a dark silhouette line in snapshots where they turn away.
+    #[test]
+    fn snapshots_draw_silhouettes_on_curved_surfaces() {
+        let mut s = Session::default();
+        s.execute("solid.sphere", &json!({"diameter": 20})).unwrap();
+        let cam = home_camera(&s);
+        let img = solvecraft_render::render(&scene(&s, &cam), &cam, 400, 400);
+        let dark = img.rgba.chunks(4).filter(|p| p[0] < 70 && p[1] < 70 && p[2] < 75).count();
+        assert!(dark > 40, "silhouette pixels: {dark}");
+    }
 }