Rarity and capacity checks

- Rarity stream picks are property-tested against the VolmLib rarity selection
- Pregen capacity growth scales the bake by the same factor as engine start

core/src/test/java/art/arcane/iris/generation/stream/RarityStreamTest.java

@@ -0,0 +1,52 @@
+package art.arcane.iris.generation.stream;
+
+import art.arcane.volmlib.util.math.Rarity;
+import art.arcane.volmlib.util.stream.ProceduralStream;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.SplittableRandom;
+
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertSame;
+
+public class RarityStreamTest {
+    @Test
+    public void picksMatchRarityStreamSelectionForEveryNoiseValue() {
+        SplittableRandom random = new SplittableRandom(4242L);
+        for (int trial = 0; trial < 200; trial++) {
+            List<Option> options = new ArrayList<>();
+            int count = 1 + random.nextInt(12);
+            for (int index = 0; index < count; index++) {
+                options.add(new Option(random.nextInt(-2, 40)));
+            }
+            double[] value = new double[1];
+            ProceduralStream<Double> noise = ProceduralStream.ofDouble((x, z) -> value[0]);
+            ProceduralStream<Option> expected = noise.selectRarity(options);
+            RarityStream<Option> candidate = new RarityStream<>(noise, options);
+            for (int sample = 0; sample < 2_000; sample++) {
+                value[0] = switch (random.nextInt(5)) {
+                    case 0 -> random.nextDouble();
+                    case 1 -> random.nextDouble(-0.5D, 1.5D);
+                    case 2 -> random.nextInt(0, 64) / 63D;
+                    case 3 -> sample % 2 == 0 ? Double.NaN : -0D;
+                    default -> Math.nextUp(random.nextInt(0, 16) / 15D);
+                };
+                assertSame(expected.get(sample, -sample), candidate.get(sample, -sample));
+            }
+        }
+    }
+
+    @Test
+    public void emptyOptionsSelectNothing() {
+        assertNull(new RarityStream<Option>(ProceduralStream.ofDouble((x, z) -> 0.5D), List.of()).get(0D, 0D));
+    }
+
+    private record Option(int rarity) implements Rarity {
+        @Override
+        public int getRarity() {
+            return rarity;
+        }
+    }
+}