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/scene/cache/properties.rs

@@ -113,6 +113,8 @@ pub fn visualization_section(entity: &EntityType) -> Option<PropSection> {
             | EntityType::Seqend(_)
             | EntityType::Leader(_)
             | EntityType::Unknown(_)
+            // Non-plotting drawing-view border: never rendered, no properties.
+            | EntityType::ViewBorder(_)
     ) {
         return None;
     }