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_set/igloos.json

@@ -0,0 +1,14 @@
+{
+  "placement": {
+    "type": "minecraft:random_spread",
+    "salt": 14357618,
+    "separation": 8,
+    "spacing": 32
+  },
+  "structures": [
+    {
+      "structure": "minecraft:igloo",
+      "weight": 1
+    }
+  ]
+}