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/village_plains.json

@@ -0,0 +1,15 @@
+{
+  "type": "minecraft:jigsaw",
+  "biomes": "#minecraft:has_structure/village_plains",
+  "max_distance_from_center": 80,
+  "project_start_to_heightmap": "WORLD_SURFACE_WG",
+  "size": 6,
+  "spawn_overrides": {},
+  "start_height": {
+    "absolute": 0
+  },
+  "start_pool": "minecraft:village/plains/town_centers",
+  "step": "surface_structures",
+  "terrain_adaptation": "beard_thin",
+  "use_expansion_hack": true
+}