The pen holder (195 KB SolidWorks STEP) tessellates in 0.3 s; the rest
of a 1.7-3.4 s cold import was the ~2.9 MB kernel download. Start the
worker when Import is clicked or a file is dragged over, so the download
overlaps with the file dialog. The vendor folder now carries the version
(occt-import-js-0.0.23) and web/_headers marks it immutable for a year.
A failed read drops the worker so the next attempt starts clean.
STEP is exact B-rep, so it goes through the Open CASCADE kernel in a
worker, tessellated at 0.01 mm chord / 0.1 rad so the faceting is below
print resolution. The assembly tree maps to the same objects the 3MF
reader returns, so a multi-body STEP uses the existing object picker.
Detected by content (ISO-10303-21 magic), like 3MF.
The top-bar button is now just "Import"; the file dialog filters to
.stl/.3mf/.step/.stp.
matthewtrahan