Separated deposit chance samples

- Clump seeds mix the generator seed and attempt index before sampling
- Deposit plans share immutable seeds across preparation workers
- Probability checks cover generator independence and adjacent clumps across batch boundaries

core/src/test/java/art/arcane/iris/engine/modifier/IrisDepositModifierContextTest.java

@@ -37,7 +37,6 @@ import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyString;
-import static org.mockito.ArgumentMatchers.anyLong;
 import static org.mockito.Mockito.RETURNS_DEEP_STUBS;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.doReturn;
@@ -103,7 +102,6 @@ public class IrisDepositModifierContextTest {
         RNG rng = mock(RNG.class);
         when(rng.d()).thenReturn(0.5D);
         when(rng.i(0, 5)).thenReturn(4, 0);
-        when(rng.nextParallelRNG(anyLong())).thenReturn(rng);
 
         new IrisDepositModifier(engine).generate(generator, null, null, rng, 0, 0, false, null);
 

core/src/test/java/art/arcane/iris/engine/modifier/IrisDepositModifierOrderingTest.java

@@ -88,7 +88,7 @@ public class IrisDepositModifierOrderingTest {
         RNG generatorRng = new RNG(0L).nextParallelRNG(0L);
         for (String name : List.of("dimension", "region", "biome")) {
             for (int i = 0; i < 25; i++) {
-                expectedSamples.add(name + ":" + generatorRng.nextParallelRNG(i + 1L).getSeed());
+                expectedSamples.add(name + ":" + IrisDepositModifier.clumpSeed(generatorRng.getSeed(), i));
             }
         }
         Collections.sort(expectedSamples);

core/src/test/java/art/arcane/iris/engine/modifier/IrisDepositModifierProbabilityTest.java

@@ -0,0 +1,36 @@
+package art.arcane.iris.engine.modifier;
+
+import art.arcane.volmlib.util.math.RNG;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class IrisDepositModifierProbabilityTest {
+    @Test
+    public void generatorAndClumpChancesRemainIndependent() {
+        int samples = 100_000;
+        int accepted = 0;
+        for (int seed = 0; seed < samples; seed++) {
+            if (new RNG(seed).d() <= 0.2D
+                    && new RNG(IrisDepositModifier.clumpSeed(seed, 0)).d() <= 0.2D) {
+                accepted++;
+            }
+        }
+        assertEquals(0.04D, accepted / (double) samples, 0.003D);
+    }
+
+    @Test
+    public void neighboringClumpChancesRemainIndependentAcrossBatchBoundaries() {
+        int samples = 100_000;
+        for (int first : new int[]{0, 7, 31}) {
+            int accepted = 0;
+            for (int seed = 0; seed < samples; seed++) {
+                if (new RNG(IrisDepositModifier.clumpSeed(seed, first)).d() <= 0.2D
+                        && new RNG(IrisDepositModifier.clumpSeed(seed, first + 1)).d() <= 0.2D) {
+                    accepted++;
+                }
+            }
+            assertEquals("Clump pair at " + first, 0.04D, accepted / (double) samples, 0.003D);
+        }
+    }
+}