Square wall corners hold grains, liquids and gas (t39)

Powder, liquid and gas no longer step diagonally between two solid
cells; flyers and diagonal tube intakes obey the same corner. World
generation settles with the rule off, so seeds make the same worlds.

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

src/sim/conveyors.js

@@ -169,6 +169,7 @@ function tubeIntake(i, p, dx, dy) {
     const oy = diagonal ? (k === 0 ? 0 : -dy) : k === 0 ? -dy : k === 1 ? dx : -dx;
     const nx = x + ox, ny = y + oy;
     if (nx < 0 || ny < 0 || nx >= W || ny >= H) continue;
+    if (ox && oy && PH[mat[y * W + nx]] === PH_SOLID && PH[mat[ny * W + x]] === PH_SOLID) continue;
     const j = ny * W + nx, m = mat[j];
     if (m === EMPTY || occ[j] || stamp[j] === tickStamp) continue;
     if (PH[m] !== PH_POWDER) continue; // the screw only takes loose material; liquids just flow through