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/world/structure_placement_test.go

@@ -0,0 +1,86 @@
+package world
+
+import (
+	"testing"
+
+	"regionio/internal/worldgen"
+)
+
+// TestStructurePlacementGrid pins the random-spread placement machinery against
+// the vanilla capture. The fixture's chunk (1,0) carries a ruined portal —
+// obsidian, crying obsidian, and a gold block around y=13-18 — and the grid
+// must claim exactly that chunk for the ruined_portals set on seed 12345.
+func TestStructurePlacementGrid(t *testing.T) {
+	sets, err := worldgen.LoadStructureSets()
+	if err != nil {
+		t.Fatal(err)
+	}
+	const seed = 12345
+	if sets.Sets["minecraft:ruined_portals"] == nil {
+		t.Fatal("ruined_portals set missing")
+	}
+	set := sets.Sets["minecraft:ruined_portals"]
+
+	var claimed []string
+	for cx := int32(-8); cx <= 8; cx++ {
+		for cz := int32(-8); cz <= 8; cz++ {
+			if set.IsStartChunk(seed, cx, cz) {
+				claimed = append(claimed, "("+structCoord(cx)+","+structCoord(cz)+")")
+			}
+		}
+	}
+	if len(claimed) != 1 || claimed[0] != "(1,0)" {
+		t.Fatalf("ruined portal starts %v, want exactly [(1,0)]", claimed)
+	}
+
+	// Mineshaft corridors from the starts around the fixture reach its deep
+	// oak-plank cells; the starts themselves sit a few chunks out.
+	mineshafts := sets.Sets["minecraft:mineshafts"]
+	near := 0
+	for cx := int32(-6); cx <= 6; cx++ {
+		for cz := int32(-6); cz <= 6; cz++ {
+			if mineshafts.IsStartChunk(seed, cx, cz) {
+				near++
+			}
+		}
+	}
+	if near == 0 {
+		t.Fatal("no mineshaft starts anywhere near the fixture")
+	}
+
+	// Villages use spacing 34/separation 8: two starts can never share a
+	// region row or column window closer than separation allows.
+	villages := sets.Sets["minecraft:villages"]
+	vp := villages.Placement.RandomSpread
+	pick := func(cx, cz int32) [2]int32 { return vp.PotentialChunk(seed, cx, cz) }
+	a := pick(0, 0)
+	b := pick(int32(vp.Spacing), 0)
+	span := int32(vp.Spacing - vp.Separation)
+	localA := a[0]
+	localB := b[0] - int32(vp.Spacing)
+	if localA < 0 || localA >= span || localB < 0 || localB >= span {
+		t.Fatalf("village offsets %v -> %v fall outside [0,%d)", a, b, span)
+	}
+	if gap := b[0] - a[0]; gap < int32(vp.Separation)+1 {
+		t.Fatalf("adjacent village regions %d apart, want >= %d", gap, vp.Separation+1)
+	}
+}
+
+func structCoord(v int32) string {
+	digits := ""
+	neg := v < 0
+	if neg {
+		v = -v
+	}
+	for v > 0 {
+		digits = string(rune('0'+v%10)) + digits
+		v /= 10
+	}
+	if digits == "" {
+		digits = "0"
+	}
+	if neg {
+		return "-" + digits
+	}
+	return digits
+}