Big DWG/DXF files block the UI for seconds during parse + cache build,
leaving the user wondering whether the app froze. Split the file-open
flow into pick → load and show a modal overlay during the load: file
name, size, phase label (Reading / Parsing / Building caches /
Finalizing), animated indeterminate bar, and Cancel button.
Phase is tracked via a shared AtomicU8 written by the loader thread
and read by the overlay each frame. Cancel hides the overlay and
discards the result; the parser thread runs to completion in the
background since acadrust has no interrupt hook.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
