Decision: WebGPU renderer and multi-core simulation in the browser

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DESIGN.md

@@ -528,6 +528,13 @@ above, including:
   rate, up to a cap per metal; power never sets hardness, so it can't skip the steel bit. It is
   tuned like the drill and never harms anything built. It waits until power is switched back on,
   since today every machine runs without power.
+- **Performance: WebGPU and all CPU cores, in the browser (2026-10-27).** Zoomed out, drawing
+  is CPU-bound (about 26 ns per visible grain) and the simulation runs on one core. The owner
+  chose: WebGPU only (no WebGL fallback), tested in Chrome and Firefox with a clear message when
+  it's missing; the game must be served with COOP/COEP headers (for SharedArrayBuffer), so
+  opening the file from disk is dropped; determinism is re-baselined once and becomes the same on
+  any number of cores; as fast as possible, measured at every step. Grain movement and heat go
+  across a worker pool first; moving the whole simulation into its own worker is decided after.
 
 ## Ideas not built yet