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/app/helpers.rs

@@ -346,6 +346,8 @@ pub(super) fn entity_type_key(entity: &acadrust::EntityType) -> String {
         AttributeEntity(_) => "attrib",
         Ole2Frame(_) => "ole2frame",
         Light(_) => "light",
+        SectionSymbol(_) => "sectionsymbol",
+        ViewBorder(_) => "viewborder",
         Seqend(_) => "seqend",
         Unknown(_) => "unknown",
     }