fix: align white paper fill with entity borders

- paper_entity_extents() computes the bounding box of actual DXF entities
  so the white fill always matches the drawn title-block / frame borders,
  regardless of whether Layout min/max_limits match entity positions
- Falls back to paper_limits() (→ A4 297×210) when layout has no entities
- Removed paper_boundary_wire: the white fill now acts as the visual paper
  edge so the near-white wire (invisible on white) was redundant
- LayoutCreate sets limits to A4 landscape immediately after add_layout()
  to override acadrust's imperial default (12×9)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/scene/paper_canvas.rs

@@ -68,10 +68,11 @@ impl<'a> canvas::Program<Message> for PaperCanvas<'a> {
         const DESK: Color = Color { r: 0.22, g: 0.24, b: 0.28, a: 1.0 };
         frame.fill_rectangle(Point::ORIGIN, bounds.size(), DESK);
 
-        // ── White paper area ──────────────────────────────────────────────────
-        if let Some(((px0, py0), (px1, py1))) = self.scene.paper_limits() {
-            let tl = to_px(px0 as f32, py1 as f32);
-            let br = to_px(px1 as f32, py0 as f32);
+        // ── White paper area — use entity extents so the fill aligns with the
+        //   drawn DXF entity borders (title block frame, viewport border, etc.).
+        if let Some(((px0, py0), (px1, py1))) = self.scene.paper_entity_extents() {
+            let tl = to_px(px0, py1);
+            let br = to_px(px1, py0);
             let pw = br.x - tl.x;
             let ph = br.y - tl.y;
             if pw > 0.0 && ph > 0.0 {