PERF.30 CadCraft: the Problems audit stringifies every entity after every change; DXF parses on the main thread; save ceilings at 1M entities

problems-cadcraft.js:45/52 shapeKey and auditIds cost 450 ms per edit at 100k entities (a JSON key per entity after each change): make the audit incremental over the touched ids. readDxf runs on the main thread (1.9 s at 100k; the DWG reader already has a worker, dwg/read-async.js): move the DXF reader into the same worker or stream it (the scale readers row). Save ceilings: writeDxf2000 dies at 1M entities with "Invalid string length", saveDwg takes 58 s and 4.2 GB of heap at 1M (over Chromium's default) and 6.1 s / 1.4 GB at 100k: chunked Blob output (the writers row) and the DWG writer in a worker (the autosave row, which measured the autosave write at 0.5 s for 10k and 5.8 s for 100k on the main thread).