Legacy hydrology tests on the v2 contract; water connects across steps

- Geometry pins became relational properties; tests for deleted mechanisms removed
- Every owned channel water column is published to the mantle so steps get fluid updates
- Upper beds at head steps reach the lower head so the water stays one body
- Ocean aprons are measured from the first ocean station and bounded laterally
- Dry cave headroom uses the flooded cave biome

core/src/main/java/art/arcane/iris/engine/hydrology/HydrologyColumnLayer.java

@@ -70,13 +70,13 @@ public record HydrologyColumnLayer(
         return surfaceBiomeKey;
     }
 
+    /** Every owned channel water column is published so steps between stations receive fluid updates. */
     public boolean publishesSurfaceFluid() {
         return feature.type().isSurface()
                 && !oceanApron
                 && channel
                 && fluidOwned
                 && (terrainOwned || fallingFluid)
-                && (feature.type().isDrop() || fallingFluid || receivingPool)
                 && fluidHeadY > bedY;
     }
 

core/src/main/java/art/arcane/iris/engine/hydrology/surface/ErosionFieldCompiler.java

@@ -78,6 +78,7 @@ public final class ErosionFieldCompiler {
                 }
             }
         }
+        int oceanStart = oceanStart(centerline, valley, terminal);
         Long2ObjectOpenHashMap<SurfaceColumn> columns = new Long2ObjectOpenHashMap<>(nearest.size());
         LongArrayList wetKeys = new LongArrayList();
         for (long key : nearest.keySet()) {
@@ -95,7 +96,8 @@ public final class ErosionFieldCompiler {
             if (!SurfaceCellAdmission.writable(terrain, seaLevel)) {
                 if (terrain != null && wet && terminal == SurfaceTerminal.OCEAN_MOUTH
                         && station >= valley.exposedStations() - 1
-                        && station - valley.exposedStations() < apronLimit) {
+                        && station - oceanStart < apronLimit
+                        && cellDistance <= apronLimit + 0.25D) {
                     columns.put(key, new SurfaceColumn(
                             cellX, cellZ, terrain, station, SurfaceRole.CHANNEL, terrain.naturalHeight(), seaLevel, true));
                 }
@@ -133,9 +135,34 @@ public final class ErosionFieldCompiler {
             columns.put(key, new SurfaceColumn(cellX, cellZ, terrain, station, SurfaceRole.BANK, height, height, false));
         }
         int uncontained = contain(columns, wetKeys, freeboard);
+        connectSteps(columns, wetKeys);
         return new ErosionField(columns, uncontained);
     }
 
+    /**
+     * Where the head steps down between neighbouring wet cells, the upper bed is lowered so its water
+     * column reaches the lower head; the water stays one connected body across every step.
+     */
+    private static void connectSteps(Long2ObjectOpenHashMap<SurfaceColumn> columns, LongArrayList wetKeys) {
+        for (int index = 0; index < wetKeys.size(); index++) {
+            long key = wetKeys.getLong(index);
+            SurfaceColumn wet = columns.get(key);
+            int bed = wet.height();
+            for (int[] offset : CARDINALS) {
+                SurfaceColumn neighbour = columns.get(RiverFootprint.pack(wet.x() + offset[0], wet.z() + offset[1]));
+                if (neighbour == null || neighbour.role() != SurfaceRole.CHANNEL || neighbour.apron()) {
+                    continue;
+                }
+                if (neighbour.headY() < wet.headY()) {
+                    bed = Math.min(bed, neighbour.headY() - 1);
+                }
+            }
+            if (bed < wet.height()) {
+                columns.put(key, wet.withBed(bed));
+            }
+        }
+    }
+
     /**
      * Every dry cell touching water, diagonals included, keeps the lip; only cardinal neighbours can
      * spill, so only they count as uncontained.
@@ -183,6 +210,21 @@ public final class ErosionFieldCompiler {
         return uncontained;
     }
 
+    /** First mouth station whose center is ocean; the apron is measured from there, not from the mouth segment start. */
+    private int oceanStart(SurfaceCenterline centerline, ValleyProfile valley, SurfaceTerminal terminal) {
+        int count = centerline.size();
+        if (terminal != SurfaceTerminal.OCEAN_MOUTH) {
+            return count;
+        }
+        for (int station = Math.max(0, valley.exposedStations() - 1); station < count; station++) {
+            HydrologyTerrainSample center = sampler.sample(centerline.x()[station], centerline.z()[station]);
+            if (!SurfaceCellAdmission.writable(center, seaLevel)) {
+                return station;
+            }
+        }
+        return count;
+    }
+
     private boolean[] basins(ValleyProfile valley, ChannelProfile channel, int count) {
         boolean[] basin = new boolean[count];
         for (int station = 1; station < count; station++) {

core/src/main/java/art/arcane/iris/engine/hydrology/surface/SurfaceColumn.java

@@ -19,7 +19,13 @@ public record SurfaceColumn(
         Objects.requireNonNull(role, "role");
     }
 
+    /** Dry columns sit at their own height; the head follows it. */
     public SurfaceColumn withHeight(int replacementHeight) {
         return new SurfaceColumn(x, z, terrain, station, role, replacementHeight, replacementHeight, apron);
     }
+
+    /** Wet columns keep their head; only the bed moves. */
+    public SurfaceColumn withBed(int replacementBed) {
+        return new SurfaceColumn(x, z, terrain, station, role, replacementBed, headY, apron);
+    }
 }

core/src/main/java/art/arcane/iris/engine/mantle/components/MantleHydrologyComponent.java

@@ -325,11 +325,8 @@ public final class MantleHydrologyComponent extends IrisMantleComponent {
                 throw new IllegalStateException("Accepted cave layer is absent from its containment plan at "
                         + position + ".");
             }
-            String biomeKey = action == HydrologyCaveAction.DRY_AIR
-                    ? layer.floodedCaveBiomeKey()
-                    : layer.floodedCaveBiomeKey();
             PlannedCell candidate = new PlannedCell(
-                    new HydrologyCaveCell(action, layer.profileKey(), biomeKey),
+                    new HydrologyCaveCell(action, layer.profileKey(), layer.floodedCaveBiomeKey()),
                     layer.feature().courseId()
             );
             mergeCaveCell(caveCells, position, candidate);