Update world shader to use material IDs

Replace bit-packed cell flags in the R channel with a direct material ID float pass-through. Food clearing now writes MAT_AIR instead of clearing a bit.

src/shaders/world.frag

@@ -12,25 +12,15 @@ void main()	{
     vec4 lastState = texture(tLastState, vUv);
     vec4 discreteAnts = texture(tDiscreteAnts, vUv);
 
-    int cellData = int(lastState.x);
-    int isFood = cellData & 1;
-    int isHome = (cellData & 2) >> 1;
-    int isObstacle = (cellData & 4) >> 2;
+    float materialId = lastState.x;
     float scentToHome = min(SCENT_MAX_PER_CELL, lastState.y + discreteAnts.x);
-    float scentToFood =  min(SCENT_MAX_PER_CELL, lastState.z + discreteAnts.y);
+    float scentToFood = min(SCENT_MAX_PER_CELL, lastState.z + discreteAnts.y);
     float repellent = min(REPELLENT_MAX_PER_CELL, lastState.w);
 
+    // ant picked up food from this cell
     if (discreteAnts.z == 1.) {
-        isFood = 0;
+        materialId = float(MAT_AIR);
     }
 
-    int terrainType = (cellData >> TERRAIN_TYPE_SHIFT) & TERRAIN_TYPE_MASK;
-    int foodQuality = (cellData >> FOOD_QUALITY_SHIFT) & FOOD_QUALITY_MASK;
-
-    // reconstruct cell data preserving all bit fields
-    int newCellData = isFood + (isHome << 1) + (isObstacle << 2)
-        + (terrainType << TERRAIN_TYPE_SHIFT)
-        + (foodQuality << FOOD_QUALITY_SHIFT);
-
-    FragColor = vec4(float(newCellData), scentToHome, scentToFood, repellent);
+    FragColor = vec4(materialId, scentToHome, scentToFood, repellent);
 }