Add ant gravity, surface constraint, and collision displacement

- isObstacle() now blocks all non-air, non-home materials so ants treat sand/dirt/rock/food as walls
- gravity pulls ants down when no solid cell is below them, gating all steering logic while falling
- collision displacement pushes ants to nearest air cell when sand falls on top of them

src/shaders/antsCompute.frag

@@ -33,12 +33,6 @@ vec2 roundUvToCellCenter(vec2 uv) {
     return floor(uv * WORLD_SIZE) / WORLD_SIZE + cellSize * 0.5;
 }
 
-bool tryGetFood(vec2 pos) {
-    float materialId = texture(tWorld, roundUvToCellCenter(pos)).x;
-
-    return int(materialId) == MAT_FOOD;
-}
-
 bool tryDropFood(vec2 pos) {
     float materialId = texture(tWorld, roundUvToCellCenter(pos)).x;
 
@@ -47,8 +41,9 @@ bool tryDropFood(vec2 pos) {
 
 bool isObstacle(vec2 pos) {
     float materialId = texture(tWorld, roundUvToCellCenter(pos)).x;
-
-    return int(materialId) == MAT_ROCK;
+    int matInt = int(materialId);
+    // ants can only move through air and home cells
+    return matInt != MAT_AIR && matInt != MAT_HOME;
 }
 
 float smell(vec2 pos, float isCarrying) {
@@ -123,6 +118,40 @@ void main()	{
         pathIntDy = 0.;
     }
 
+    // --- GRAVITY AND SURFACE CHECK ---
+    vec2 cellCenter = roundUvToCellCenter(pos);
+    float belowMat = texture(tWorld, cellCenter - vec2(0., cellSize)).x;
+    float currentMat = texture(tWorld, cellCenter).x;
+
+    bool belowSolid = (int(belowMat) != MAT_AIR);
+
+    // collision displacement: if current cell is now solid (sand fell on us), push to nearest air
+    if (int(currentMat) != MAT_AIR && int(currentMat) != MAT_HOME) {
+        vec2 upPos = cellCenter + vec2(0., cellSize);
+        vec2 leftPos = cellCenter - vec2(cellSize, 0.);
+        vec2 rightPos = cellCenter + vec2(cellSize, 0.);
+        if (int(texture(tWorld, upPos).x) == MAT_AIR) {
+            pos = upPos;
+        } else if (int(texture(tWorld, leftPos).x) == MAT_AIR) {
+            pos = leftPos;
+        } else if (int(texture(tWorld, rightPos).x) == MAT_AIR) {
+            pos = rightPos;
+        }
+        cellCenter = roundUvToCellCenter(pos);
+        belowMat = texture(tWorld, cellCenter - vec2(0., cellSize)).x;
+        belowSolid = (int(belowMat) != MAT_AIR);
+    }
+
+    bool isFalling = false;
+
+    // GRAVITY: if nothing solid below and not at world bottom, fall
+    if (!belowSolid && pos.y > cellSize) {
+        pos.y -= cellSize;
+        isFalling = true;
+    }
+
+    if (!isFalling) {
+
     if (isCarrying == 0.) {
         if (noise < 0.33) {
             vec2 offset = vec2(cos(angle), sin(angle)) * sampleDistance;
@@ -198,8 +227,8 @@ void main()	{
 
             // 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);
+            float belowDigMat = texture(tWorld, belowUv).x;
+            vec4 belowProps = texelFetch(uMaterialProps, ivec2(int(belowDigMat), 0), 0);
 
             if (belowProps.r == BEHAVIOR_POWDER && belowProps.b <= ANT_CARRY_STRENGTH) {
                 // diggable material below — bias downward
@@ -293,6 +322,8 @@ void main()	{
         }
     }
 
+    } // end !isFalling
+
     FragColor = vec4(
         pos.x,
         pos.y,