Point basis samples carry terrain slope

Routing, validation and the valley solver sampled basis columns with slope 0; the forward slope now rides on every point sample.

core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSampler.java

@@ -106,7 +106,9 @@ final class IrisHydrologyRoutingTerrainSampler implements HydrologyNaturalTerrai
 
     @Override
     public HydrologyTerrainSample sampleBasis(int blockX, int blockZ) {
-        return basis(blockX, blockZ).terrain();
+        // Point samples carry the forward slope; the aligned grid derives its own from neighbouring bases.
+        TerrainBasis basis = basis(blockX, blockZ);
+        return basis.terrain().withSlope(localSlope(blockX, blockZ, basis.naturalHeight()));
     }
 
     TerrainBasis basis(int blockX, int blockZ) {

core/src/main/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRuntime.java

@@ -387,9 +387,7 @@ public final class IrisHydrologyRuntime implements AutoCloseable {
     }
 
     private HydrologyTerrainSample createDetailedTerrainSample(int x, int z) {
-        IrisHydrologyRoutingTerrainSampler.TerrainBasis basis = routingTerrainSampler.basis(x, z);
-        double slope = routingTerrainSampler.localSlope(x, z, basis.naturalHeight());
-        return basis.terrain().withSlope(slope);
+        return routingTerrainSampler.sampleBasis(x, z);
     }
 
     private IrisHydrologyRoutingTerrainSampler.TerrainBasis createTerrainBasis(

core/src/test/java/art/arcane/iris/engine/hydrology/runtime/IrisHydrologyRoutingTerrainSamplerTest.java

@@ -93,6 +93,24 @@ public class IrisHydrologyRoutingTerrainSamplerTest {
         }
     }
 
+    @Test
+    public void pointBasisSamplesCarryTheForwardSlope() {
+        IrisHydrologyRoutingTerrainSampler sampler = sampler(64);
+        int[][] points = {{0, 0}, {-129, -257}, {37, 1024}};
+
+        for (int[] point : points) {
+            int x = point[0];
+            int z = point[1];
+            double expected = IrisHydrologyRoutingTerrainSampler.localSlope(
+                    height(x, z), height(x + 3, z), height(x, z + 3));
+
+            double actual = sampler.sampleBasis(x, z).slope();
+
+            assertTrue(expected > 0D);
+            assertEquals(Double.doubleToRawLongBits(expected), Double.doubleToRawLongBits(actual));
+        }
+    }
+
     @Test
     public void adjacentGridsShareBasesAndProduceIdenticalOverlap() {
         AtomicInteger calls = new AtomicInteger();