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/render/src/raster.rs

@@ -113,6 +113,9 @@ pub fn render(scene: &Scene, cam: &Camera, w: usize, h: usize) -> Canvas {
                 continue;
             };
             let ns: Vec<Vec3> = t.iter().map(|k| m.normals.get(*k as usize).copied().unwrap_or(Vec3::Z)).collect();
+            // A curved surface (its normals turn across the triangle) gets a silhouette line
+            // where it turns away from the eye.
+            let curved = ns.iter().any(|n| n.dist(ns[0]) > 1e-6);
             let (x0, x1) = (a[0].min(b[0]).min(cc[0]).floor().max(0.0), a[0].max(b[0]).max(cc[0]).ceil().min(sw as f64 - 1.0));
             let (y0, y1) = (a[1].min(b[1]).min(cc[1]).floor().max(0.0), a[1].max(b[1]).max(cc[1]).ceil().min(sh as f64 - 1.0));
             if !(x0 <= x1 && y0 <= y1) {
@@ -138,7 +141,15 @@ pub fn render(scene: &Scene, cam: &Camera, w: usize, h: usize) -> Canvas {
                             *d = z;
                         }
                         let n = (ns[0] * w0 + ns[1] * w1 + ns[2] * w2).normalized().unwrap_or(Vec3::Z);
-                        c.put(x, y, shade(n, view_dir, base), 1.0);
+                        let mut col = shade(n, view_dir, base);
+                        let ndv = n.dot(view_dir).abs();
+                        if curved && ndv < 0.14 {
+                            let a = 1.0 - ndv / 0.14;
+                            for (k, e) in col.iter_mut().zip([0.07, 0.08, 0.10]) {
+                                *k = *k * (1.0 - a) + e * a;
+                            }
+                        }
+                        c.put(x, y, col, 1.0);
                     }
                 }
             }