fix(paper): handle NaN sentinels when building hatch fill path

`HatchModel.boundary` stores multi-region hatches as a single Vec
with NaN-NaN sentinels separating each disconnected sub-path
(documented on the field). The paper-canvas hatch fill ignored the
sentinels and forwarded every vertex straight to the iced canvas
path builder. When a layout contained a multi-region hatch, lyon
received a polygon containing NaN coordinates and panicked with
`Tessellate path.: Internal(IncorrectActiveEdgeOrder(4))` from
inside `iced_wgpu::geometry::Frame::fill`, taking the whole app down
the first time Layout 1 painted.

Walk the boundary instead, opening a fresh sub-path on each sentinel
and closing whichever was active. Each region is now filled as its
own closed polygon. Reproduces with #22.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/scene/paper_canvas.rs

@@ -174,13 +174,32 @@ fn draw_hatch(frame: &mut canvas::Frame, hatch: &HatchModel, to_px: &impl Fn(f32
     // Reconstruct offset-rel WCS from stored small offsets + f64 anchor.
     let ox = hatch.world_origin[0] as f32;
     let oy = hatch.world_origin[1] as f32;
+    // `boundary` may carry multiple disconnected sub-paths separated by
+    // NaN-NaN sentinels (multi-region hatches with islands / holes).
+    // Forwarding NaN to lyon's path builder panics
+    // (`IncorrectActiveEdgeOrder`) — start a fresh sub-path on each
+    // sentinel so each region is closed and filled independently.
     let path = canvas::Path::new(|builder| {
-        let first = to_px(hatch.boundary[0][0] + ox, hatch.boundary[0][1] + oy);
-        builder.move_to(first);
-        for &[x, y] in &hatch.boundary[1..] {
-            builder.line_to(to_px(x + ox, y + oy));
+        let mut active = false;
+        for &[x, y] in hatch.boundary.iter() {
+            if x.is_nan() || y.is_nan() {
+                if active {
+                    builder.close();
+                    active = false;
+                }
+                continue;
+            }
+            let p = to_px(x + ox, y + oy);
+            if !active {
+                builder.move_to(p);
+                active = true;
+            } else {
+                builder.line_to(p);
+            }
+        }
+        if active {
+            builder.close();
         }
-        builder.close();
     });
 
     match &hatch.pattern {