refactor(dwg): use the first-class SectionSymbol and ViewBorder entities

Section marks and drawing-view borders are now real acadrust entity
types instead of Unknown-with-extras, so the renderer dispatches on the
variant directly, the arrow-direction resolver reads the active viewport
off the ViewBorder entity, and selection / properties show "Section
Symbol" and "View Border" instead of "Unknown". View borders are
non-plotting and tessellate to nothing; both count toward the lossy
Save-As warning since they round-trip via their raw records.

Bumps acadrust for the entity promotion.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entities/traits.rs

@@ -143,6 +143,8 @@ pub fn entity_type_name(et: &EntityType) -> &'static str {
         EntityType::Ole2Frame(_) => "Ole2Frame",
         EntityType::PolygonMesh(_) => "PolygonMesh",
         EntityType::Light(_) => "Light",
+        EntityType::SectionSymbol(_) => "SectionSymbol",
+        EntityType::ViewBorder(_) => "ViewBorder",
         EntityType::Unknown(_) => "Unknown",
     }
 }