Reworked terrain transitions

- New terrain heights meet saved boundaries while retaining local relief
- Surface water levels approach saved fluid heights across the band
- Cave and physical biome matching use a four-block seam patch
- Terrain generates once without speculative neighbor containment
- Bounded caches serve transition membership and boundary lookups
- Saved biome writes honor the selected durability policy
- Release labels and presentation code leave generation identity unchanged
- Unfinished objects retain blocks and metadata across pack changes
- Whole structure assemblies preserve pending excavation and foundations
- New placements keep pending historical pieces clear
- Transition probes cover cutovers and retained object fragments

probe/src/test/resources/transition-packs/boundary-structure/biomes/terrain.json

@@ -0,0 +1,21 @@
+{
+  "name": "Terrain",
+  "derivative": "minecraft:plains",
+  "vanillaDerivative": "minecraft:plains",
+  "layers": [
+    {
+      "palette": [
+        {
+          "block": "minecraft:stone"
+        }
+      ]
+    }
+  ],
+  "generators": [
+    {
+      "generator": "flat",
+      "min": 32,
+      "max": 32
+    }
+  ]
+}