PERF.9 CadCraft: opening an 8 MB DWG takes 4.1 s with one 1.3 s freeze when the worker hands the drawing back

Measured 2026-11-08 in /use (tools/perf/open-file.mjs): testfiles/powerline-plans/Plans.dwg (8.2 MB, 288 objects, 50 blocks, 2 layouts): quiet after 4.1 s; the reader runs in a worker (dwg/read-async.js, 1.8 s in Node) and then ONE long task of 1,258 ms on the page (cadcraft.js:2414 after readDwgAsync: building the drawing, blocks, display lists, first regen); heap 19 MB to 174 MB (peak 250 MB).

Plan: build the entity records and the display lists in the worker (structured-clone cost vs transferable arrays: measure), or chunk the page-side build with a yield every 50 ms and paint what is there; keep the first regen under 200 ms. The autosave write (dwg-save.js, PERF.5) is the other half of the same drawing's cost.