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>
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