PERF.21 Scale: ModelCraft, DetailCraft, PartCraft and CivilCraft snapshot the whole document as JSON on every undo step and autosave

Measured: a 400 MB glTF (5 M faces) imports into ModelCraft in 17 s and 2.5 GB, then the FIRST edit calls model.js:2408 snap() (also :2223, :2230, :2272, :2331): 407 MB of JSON, 35.6 s, 9 GB peak, so the tab dies after a successful import. At 500k faces the snapshot is 3.1 s per edit. Same shape in DetailCraft model.js:169-199 (stringify before AND after every change), PartCraft part.js:504-517 and CivilCraft project.js:155. Change: command or diff undo (or per-definition dirty snapshots), meshes kept as typed buffers instead of base64 inside JSON, autosave payloads as Blobs of those buffers (the shell's IndexedDB store already takes Blobs). ModelCraft first. Extends PERF.7 with the 10x and 100x numbers.