fix(block_cache): drop zero-bbox placeholders from defn LO scan

`Entity::bounding_box()` for empty / vertex-less entities (Polyline,
LwPolyline, Hatch, etc.) returns `BoundingBox3D::from_point(Vector3::ZERO)` —
the (0, 0, 0)→(0, 0, 0) placeholder. Including those in the LO centroid
scan dragged it toward the origin even for UTM-authored blocks, defeating
the precision-preservation: stored f32 then sat at UTM-minus-near-zero,
losing precision before tessellate even returned. Reject the zero
placeholder so only real geometry contributes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/block_cache.rs

@@ -285,6 +285,19 @@ fn build_defn(
                 (bb.min, bb.max)
             }
         };
+        // Skip the "placeholder ZERO bbox" that empty Polyline / Hatch /
+        // Spline / Mesh / etc. return when they have no vertices —
+        // including these would pull the centroid toward origin and ruin
+        // the precision-preservation for UTM-authored content.
+        let is_zero_placeholder = bmin.x == 0.0
+            && bmin.y == 0.0
+            && bmin.z == 0.0
+            && bmax.x == 0.0
+            && bmax.y == 0.0
+            && bmax.z == 0.0;
+        if is_zero_placeholder {
+            continue;
+        }
         let lo_arr = [bmin.x, bmin.y, bmin.z];
         let hi_arr = [bmax.x, bmax.y, bmax.z];
         for i in 0..3 {