Fix ant physics: pickup, gravity, spawn angle, food drop

- Pickup checks cell ahead instead of current cell (ants walk in
  air and can't enter solid cells, so current cell was always air)
- Starting ants face randomly left/right instead of straight down
  into sand where they'd get stuck bouncing
- Gravity falls up to 4 cells/frame instead of 1 (was imperceptibly
  slow at worldSize=1024)
- tryDropFood checks cell below too so ants walking above home can
  still deposit food

src/shaders/antsCompute.frag

@@ -35,9 +35,9 @@ vec2 roundUvToCellCenter(vec2 uv) {
 }
 
 bool tryDropFood(vec2 pos) {
-    float materialId = texture(tWorld, roundUvToCellCenter(pos)).x;
-
-    return int(materialId) == MAT_HOME;
+    float currentMatId = texture(tWorld, roundUvToCellCenter(pos)).x;
+    float belowMatId = texture(tWorld, roundUvToCellCenter(pos - vec2(0., cellSize))).x;
+    return int(currentMatId) == MAT_HOME || int(belowMatId) == MAT_HOME;
 }
 
 bool isObstacle(vec2 pos) {
@@ -145,7 +145,7 @@ void main()	{
                 }
             }
             pos = vec2(spawnX, surfaceY);
-            angle = -PI * 0.5; // face downward initially
+            angle = (rand(vUv * 31337.) > 0.5) ? 0.0 : PI; // face randomly left or right
             #else
             pos = vec2(0.5);
             angle = rand(vUv * 10000.) * 2. * PI;
@@ -185,9 +185,14 @@ void main()	{
 
     bool isFalling = false;
 
-    // GRAVITY: if nothing solid below and not at world bottom, fall
+    // GRAVITY: if nothing solid below and not at world bottom, fall multiple cells
     if (!belowSolid && pos.y > cellSize) {
-        pos.y -= cellSize;
+        for (int g = 0; g < 4; g++) {
+            if (pos.y <= cellSize) break;
+            float matBelow = texture(tWorld, roundUvToCellCenter(pos - vec2(0., cellSize))).x;
+            if (int(matBelow) != MAT_AIR) break;
+            pos.y -= cellSize;
+        }
         isFalling = true;
     }
 
@@ -326,23 +331,24 @@ void main()	{
             angle += PI * (noise - 0.5);
         }
 
-        // ---- PICKUP ----
+        // ---- PICKUP (check cell ahead — ants walk in air, can't enter solid cells) ----
         if (isCarrying == 0.) {
-            float cellMatId = texture(tWorld, roundUvToCellCenter(pos)).x;
-            int cellMatInt = int(cellMatId);
+            vec2 aheadUv = roundUvToCellCenter(pos + vec2(cos(angle), sin(angle)) * cellSize);
+            float aheadMatId = texture(tWorld, aheadUv).x;
+            int aheadMatInt = int(aheadMatId);
 
-            if (cellMatInt == MAT_FOOD) {
+            if (aheadMatInt == MAT_FOOD) {
                 isCarrying = 1.;
-                cargoMaterialId = cellMatId;
+                cargoMaterialId = aheadMatId;
                 angle += PI;
                 storage = getMaxScentStorage(vUv);
-            } else if (cellMatInt != MAT_AIR && cellMatInt != MAT_HOME) {
-                vec4 props = texelFetch(uMaterialProps, ivec2(cellMatInt, 0), 0);
+            } else if (aheadMatInt != MAT_AIR && aheadMatInt != MAT_HOME) {
+                vec4 props = texelFetch(uMaterialProps, ivec2(aheadMatInt, 0), 0);
                 float behavior = props.r;
                 float hardness = props.b;
                 if (behavior == BEHAVIOR_POWDER && hardness <= ANT_CARRY_STRENGTH) {
                     isCarrying = 1.;
-                    cargoMaterialId = cellMatId;
+                    cargoMaterialId = aheadMatId;
                     angle += PI;
                     storage = getMaxScentStorage(vUv);
                 }