worldgen: parse structure sets and place them on the vanilla grid

Loads the 26.1.2 structure datapack: 20 structure sets, 34 structure
definitions, and the worldgen biome tags they reference. Random-spread
placement follows RandomSpreadStructurePlacement byte for byte — region
floorDiv by spacing, one Legacy stream salted per region, two
spread-type draws for the offset — and the four legacy frequency
reducers keep their exact draws, including the hardcoded buried-treasure
salt and the outpost's discarded full-range int.

Verified against the fixture: on seed 12345 the ruined_portals set
claims exactly chunk (1,0), the chunk whose captured blocks carry the
ruined portal's obsidian, gold, and crying obsidian. TestStructure-
PlacementGrid pins that plus village separation invariants.

internal/worldgen/data/structure_json/fortress.json

@@ -0,0 +1,42 @@
+{
+  "type": "minecraft:fortress",
+  "biomes": "#minecraft:has_structure/nether_fortress",
+  "spawn_overrides": {
+    "monster": {
+      "bounding_box": "piece",
+      "spawns": [
+        {
+          "type": "minecraft:blaze",
+          "maxCount": 3,
+          "minCount": 2,
+          "weight": 10
+        },
+        {
+          "type": "minecraft:zombified_piglin",
+          "maxCount": 4,
+          "minCount": 4,
+          "weight": 5
+        },
+        {
+          "type": "minecraft:wither_skeleton",
+          "maxCount": 5,
+          "minCount": 5,
+          "weight": 8
+        },
+        {
+          "type": "minecraft:skeleton",
+          "maxCount": 5,
+          "minCount": 5,
+          "weight": 2
+        },
+        {
+          "type": "minecraft:magma_cube",
+          "maxCount": 4,
+          "minCount": 4,
+          "weight": 3
+        }
+      ]
+    }
+  },
+  "step": "underground_decoration"
+}