Add gravity-aware digging direction for ants

In side-view mode, ants carrying non-food powder material now bias their
angle upward toward the surface (30% blend). Ants not carrying anything
bias downward (20% blend) when diggable powder material is detected below.
Both biases are subtle so pheromone-following still shapes overall behavior.

src/shaders/antsCompute.frag

@@ -180,6 +180,38 @@ void main()	{
     }
 
     if (!movementProcessed) {
+        #if VIEW_MODE_SIDE
+        // vertical bias for digging behavior
+        if (isCarrying == 1. && int(cargoMaterialId) != MAT_FOOD) {
+            // carrying powder: bias upward toward surface
+            float upwardBias = PI * 0.5; // straight up
+            float angleDiff = upwardBias - angle;
+            // normalize to [-PI, PI]
+            angleDiff = mod(angleDiff + PI, 2.0 * PI) - PI;
+            // gentle steering toward up (blend 30% toward upward)
+            angle += angleDiff * 0.3;
+        } else if (isCarrying == 0.) {
+            // not carrying: check if surrounded by diggable material
+            // if so, prefer downward movement
+            float cellMatId = texture(tWorld, roundUvToCellCenter(pos)).x;
+            vec4 props = texelFetch(uMaterialProps, ivec2(int(cellMatId), 0), 0);
+
+            // check cell below for diggable material
+            vec2 belowUv = roundUvToCellCenter(pos - vec2(0., cellSize));
+            float belowMat = texture(tWorld, belowUv).x;
+            vec4 belowProps = texelFetch(uMaterialProps, ivec2(int(belowMat), 0), 0);
+
+            if (belowProps.r == BEHAVIOR_POWDER && belowProps.b <= ANT_CARRY_STRENGTH) {
+                // diggable material below — bias downward
+                float downwardBias = -PI * 0.5; // straight down
+                float angleDiff = downwardBias - angle;
+                angleDiff = mod(angleDiff + PI, 2.0 * PI) - PI;
+                // gentle steering toward down (20% blend)
+                angle += angleDiff * 0.2;
+            }
+        }
+        #endif
+
         float noise2 = rand(vUv * 1000. + fract(uTime / 1000.) + 0.2);
 
         float sampleAhead = smell(applyOffsetToPos(pos, vec2(cos(angle), sin(angle)) * sampleDistance), isCarrying);