Buildings cast no shadows: daylight passes player-built pieces (t37)

The lighting finds its own roof per column, skipping walls, belts, tubes, machines and molds,
and lets light through them like air, in both looks. Natural rock still shades; the
simulation's roof (steam condensing under a built roof) is unchanged. New test/t_light.js.

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

src/render/light.js

@@ -2,6 +2,9 @@
 // ---------- Lighting ----------
 // Light is computed on a coarse grid (2 or 4 grains per cell), written into a
 // small canvas, then the GPU upscales it smoothly and multiplies it over the world.
+function builtForLight(i, x, y) {
+  return (PH[mat[i]] === PH_SOLID && !(flags[i] & F_NAT)) || blockP[(y >> 2) * BW + (x >> 2)] || mach[i] || part[i];
+}
 function computeLight() {
   const res = SET.lighting === 'fast' ? 4 : 2;
   const M = Math.ceil(28 / res);
@@ -13,6 +16,7 @@ function computeLight() {
     R.lightC.width = lw; R.lightC.height = lh;
     R.lightImg = R.lightX.createImageData(lw, lh);
     R.lightPx = new Uint32Array(R.lightImg.data.buffer);
+    R.lightRoof = new Int32Array(lw);
   }
   R.lx0 = lx0; R.ly0 = ly0;
   const lr = R.lr, lg = R.lg, lb = R.lb, la = R.latt;
@@ -20,15 +24,27 @@ function computeLight() {
   const P = res;
   const aAir = Math.pow(0.978, P), aGas = Math.pow(0.96, P), aLiq = Math.pow(0.93, P), aSol = Math.pow(0.9, P);
   const skyK = SET.daylight * (1 - 0.32 * (R.rain || 0)); // overcast while it rains
+  // Only sampled view columns, starting at the simulation roof; nothing below the view matters.
+  const roofEnd = Math.min(H, (ly0 + lh) * res), lightRoof = R.lightRoof;
+  for (let lx = 0; lx < lw; lx++) {
+    const x = (lx0 + lx) * res;
+    if (x < 0 || x >= W) continue;
+    let y = roofY[x];
+    for (; y < roofEnd; y++) {
+      const i = y * W + x, ph = PH[mat[i]];
+      if ((ph === PH_SOLID || ph === PH_POWDER || ph === PH_LIQUID) && !builtForLight(i, x, y)) break;
+    }
+    lightRoof[lx] = y;
+  }
   for (let ly = 0; ly < lh; ly++) {
     const wy = (ly0 + ly) * res;
     for (let lx = 0; lx < lw; lx++) {
       const wx = (lx0 + lx) * res, k = ly * lw + lx;
       if (wx < 0 || wy < 0 || wx >= W || wy >= H) { la[k] = 0; lr[k] = lg[k] = lb[k] = 0; continue; }
       const i = wy * W + wx, m = mat[i], ph = PH[m];
-      la[k] = ph === PH_EMPTY ? (bgm[i] ? aGas : aAir) : ph === PH_GAS ? aGas : ph === PH_LIQUID ? aLiq : m === GLASS || m === ICE ? aGas : aSol;
+      la[k] = builtForLight(i, wx, wy) ? aAir : ph === PH_EMPTY ? (bgm[i] ? aGas : aAir) : ph === PH_GAS ? aGas : ph === PH_LIQUID ? aLiq : m === GLASS || m === ICE ? aGas : aSol;
       let r = 0, g = 0, b = 0;
-      if (wy < roofY[wx]) { r = skyK; g = skyK * 0.98; b = skyK * 0.94; }
+      if (wy < lightRoof[lx]) { r = skyK; g = skyK * 0.98; b = skyK * 0.94; }
       else if (wy < surfY[wx] && ph !== PH_SOLID) { const v = skyK * 0.9; r = v; g = v * 0.98; b = v * 0.95; }
       // emission from heat: hottest grain in the block
       let T = 0;

src/render/world.js

@@ -219,7 +219,7 @@ function drawWorld() {
           else cl = 5;
         }
         top = cl << 24;
-        fpx[row + bx] = (tb ? 0xff000000 : 0) | (cl === 5 ? 0xff0000 : 0) | (cl === 3 ? 0xff00 : 0) | (cl === 1 || cl === 4 || cl === 6 ? 0xff : 0);
+        fpx[row + bx] = (tb ? 0xff000000 : 0) | (cl === 5 ? 0xff0000 : 0) | (cl === 3 ? 0xff00 : 0) | (cl === 1 && !builtForLight(i, x, y) ? 0xff : 0);
       }
       if (fv < 255) {
         const k = fv / 255; r *= k; g *= k; b *= k;