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(