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/slope-mountain/dimensions/transition.json

@@ -0,0 +1,20 @@
+{
+  "name": "Mountain transition fixture",
+  "version": 1,
+  "mode": {
+    "type": "OVERWORLD"
+  },
+  "regions": [
+    "terrain"
+  ],
+  "environment": "NORMAL",
+  "dimensionHeight": {
+    "min": 0,
+    "max": 128
+  },
+  "logicalHeight": 128,
+  "fluidHeight": 40,
+  "caveProfile": {
+    "enabled": false
+  }
+}