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/end_cities.json

@@ -0,0 +1,15 @@
+{
+  "placement": {
+    "type": "minecraft:random_spread",
+    "salt": 10387313,
+    "separation": 11,
+    "spacing": 20,
+    "spread_type": "triangular"
+  },
+  "structures": [
+    {
+      "structure": "minecraft:end_city",
+      "weight": 1
+    }
+  ]
+}