Gate ant initialization by ANTS_START_COUNT budget

Ants whose index exceeds ANTS_START_COUNT output zero state and return
immediately on init, keeping them dormant without consuming GPU work.
This lets the ant texture capacity exceed the active ant count without
spawning unwanted ants.

src/shaders/antsCompute.frag

@@ -91,7 +91,19 @@ void main()	{
 
     bool movementProcessed = false;
 
+    // calculate this ant's index from its UV position in the texture
+    ivec2 texSize = textureSize(tLastState, 0);
+    int antIndex = int(vUv.y * float(texSize.y)) * texSize.x + int(vUv.x * float(texSize.x));
+
     if (pos == vec2(0)) { // init new ant
+        // only activate ants within the start count
+        if (antIndex >= ANTS_START_COUNT) {
+            // dormant ant — output zeros and skip everything
+            FragColor = vec4(0);
+            FragColorExt = vec4(0);
+            return;
+        }
+
         #if VIEW_MODE_SIDE
         // spawn on sand surface: random X, scan down from top to find first non-air cell
         float spawnX = rand(vUv * 10000.);