Lighting on the GPU in the WebGPU path (t88 phase 2b)

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

tools/scenes/t88-light.js

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+// Frozen light comparisons, also used by the WebGPU browser regression suite.
+window.lightScene = kind => {
+  const cx = (W / 2) & ~3, cy = (H / 2) & ~3;
+  window.CAM = [cx, cy];
+  SET.fog = false; SET.shake = false; SET.cameraLead = false;
+  SET.daylight = kind === 'fire' ? 0 : 1; SET.ambient = 0.12;
+  bot.lamp = kind === 'lamp'; weather.next = 1e15; R.rain = 0;
+  const x0 = Math.max(0,cx-550), x1 = Math.min(W,cx+550);
+  const y0 = Math.max(0,cy-330), y1 = Math.min(H,cy+330);
+  for(let y=y0;y<y1;y++) for(let x=x0;x<x1;x++) {
+    const i=y*W+x;
+    const cave = x>cx-95 && x<cx+95 && y>cy-38 && y<cy+36;
+    const opening = kind==='opening' && x>=cx+24 && x<cx+40 && y<cy;
+    setCell(i, cave || opening || y<cy-100 ? EMPTY : STONE,20,F_NAT);
+    bgm[i]=part[i]=mach[i]=occ[i]=0; blockP[(y>>2)*BW+(x>>2)]=0;
+  }
+  for(let x=x0;x<x1;x++) { surfY[x]=cy-100; dirtY[x]=cy-90; graniteY[x]=cy+100; basaltY[x]=cy+200; }
+  if(kind==='magma' || kind==='fire') {
+    for(let y=cy+15;y<cy+23;y++) for(let x=cx-25;x<cx-8;x++) setCell(y*W+x,kind==='magma'?MAGMA:FIRE,kind==='magma'?1450:1100,F_NAT);
+  }
+  // A ledge makes occlusion/turning light paths visible around each source.
+  for(let y=cy-5;y<cy+7;y++) for(let x=cx+12;x<cx+45;x++) setCell(y*W+x,STONE,20,F_NAT);
+  updateRoof(); buildSky();
+};
+lightScene('lamp');