fix: identify DWG sheet viewport by center=(0,0) instead of scale heuristic

Debug output revealed the sheet viewport has id=0 (DWG reader never sets
the field), scale=0.8965 (not ≈1.0, so the 0.02 tolerance missed it),
and center=(0.0, 0.0) — exactly at the paper-space origin.

In DWG format the overall/sheet viewport always has its center at the
origin. Content viewports are placed at their actual paper positions and
always have a non-zero center. Switching from the scale heuristic to the
center-position check makes the detection reliable regardless of what
scale value the sheet viewport happens to carry.

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

src/scene/mod.rs

@@ -209,20 +209,15 @@ impl Scene {
         if vp.id == 1 { return false; }
         if vp.id > 1  { return true;  }
         // id ≤ 0: DWG files never write group-code 69 (viewport id), so all
-        // viewports arrive with id=0. Distinguish the sheet viewport by scale:
-        // the sheet ("overall") viewport has view_height ≈ vp.height → scale ≈ 1.
-        // True content viewports have deliberate drawing scales (1:50, 1:100 …)
-        // whose scale value is far from 1.0. The old check also required the
-        // center to be at the origin, but the sheet viewport center is the
-        // paper centre (e.g. 105, 148.5 for A4), so that condition failed for
-        // DWG files and incorrectly classified the sheet viewport as a content
-        // viewport — making it tessellated and enterable via double-click.
-        let scale = if vp.view_height.abs() > 1e-9 {
-            vp.height / vp.view_height
-        } else {
-            1.0
-        };
-        (scale - 1.0).abs() >= 0.02
+        // viewports arrive with id=0.
+        //
+        // In DWG format the sheet ("overall") viewport always has its center at
+        // the paper-space origin (0, 0). Content viewports are placed at their
+        // actual position on the paper and therefore have a non-zero center.
+        // Using center position is more reliable than a scale heuristic because
+        // the sheet viewport's scale is not always exactly 1:1 (observed: 0.8965
+        // in real-world files, which the old 0.02 tolerance missed entirely).
+        vp.center.x.abs() >= 0.5 || vp.center.y.abs() >= 0.5
     }