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/io/mod.rs

@@ -338,7 +338,12 @@ pub fn dropped_on_save_count(
         })
         .count();
     for e in doc.entities() {
-        if matches!(e, acadrust::EntityType::Unknown(_)) {
+        if matches!(
+            e,
+            acadrust::EntityType::Unknown(_)
+                | acadrust::EntityType::SectionSymbol(_)
+                | acadrust::EntityType::ViewBorder(_)
+        ) {
             n += 1;
         }
     }