Multi-core simulation: grain movement and heat across a worker pool (t88 phase 3)

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

src/ui/settings.js

@@ -29,6 +29,7 @@ const SET_TABS = {
     { note: 'Upscaling (DLSS, FSR), frame generation and hardware ray tracing need native graphics APIs that browsers do not expose. They are planned for the compiled build.' },
   ],
   Simulation: [
+    { k: 'workerCap', label: 'CPU workers', d: 'Maximum helpers for grains and heat. Applies when you next load a world.', type: 'range', min: 1, max: 16, step: 1, fmt: v => v },
     { k: 'simSpeed', label: 'Simulation speed', type: 'select', opts: [[0.5, '0.5×'], [1, '1×'], [2, '2×']] },
     { k: 'maxSteps', label: 'Catch-up steps per frame', d: 'How hard the game tries to keep real time when frames are slow.', type: 'range', min: 1, max: 4, step: 1, fmt: v => v },
     { note: 'The whole world keeps simulating, wherever you are. Your factories keep running when you fly away. Areas where nothing moves or changes temperature go to sleep and cost almost nothing; busy areas cost the same near you or far away.' },