PERF.24 Scale: autosave rewrites the whole document every couple of seconds of editing, in every app (extends PERF.5)

Measured and read: CadCraft autosave writes a full DWG (dwg-autosave.js:39 saveDwg; 4.8 s today, 66 s at 10x), BimCraft the full IFC bytes (bimcraft.js:585 to model.js save), MarkupCraft the whole PDF Blob (markupcraft.js:4715; TakeoffCraft stops at 64 MB, takeoffcraft.js:4313), ModelCraft the JSON (modelcraft.js:3883). On a big file autosave is the longest task the person sees while working. Change: journal autosave, keeping the opened file's bytes or handle plus the edits map (IfcFile already holds exactly that in memory), snapshots taken in a worker from transferred buffers, and a clear cap with a message above a size. Shared piece in the shell's app-autosave.js, one snapshot function per app.