PERF.39 PartCraft: a rebuild is one synchronous task (750 features 3.0 s, a 40 by 40 hole plate 8.9 s, every edit 1.3 to 1.6 s) and the D7.51 checkpoints save nothing

Measured in /use 0.8.1 (tools/perf/apps3d-partcraft.mjs): rebuilds are linear (4 to 8 ms per feature, 5.6 ms per hole: 50 features 0.32 s, 200 1.6 s, 750 3.0 s; plates 10x10 0.43 s, 20x20 1.5 s where the 0.5 JS died, 30x30 3.7 s, 40x40 8.9 s), but rebuild() (partcraft.js:351 into the manifold wasm) runs on the page thread as one task, so the app freezes for the whole of it, and an edit of the LAST feature with the checkpoint costs 1,475 ms against 1,352 ms for a full rebuild because prefixKeys (kernel.js:814) hashes every feature's JSON per rebuild. Change: the mesh kernel in a worker like brep-worker.js with progress and cancel, cheap prefix hashing (a per-feature hash kept on the feature), undo and redo (1.6 and 1.3 s today) from cached results. Healthy: orbit at 15 draw calls, the Parameters window 5 ms.