Pin feature ordering to vanilla runtime

internal/world/biome_lookup.go

@@ -62,6 +62,7 @@ func depthRange(v any) (worldgen.ClimateRange, bool) {
 var (
 	biomeTable     *worldgen.ParameterTable
 	biomeTableOnce sync.Once
+	biomeOrder     []string
 )
 
 // loadBiomeTable parses the embedded biome parameters once and returns the full
@@ -76,18 +77,28 @@ func loadBiomeTable() *worldgen.ParameterTable {
 			panic(fmt.Sprintf("world: parsing embedded biome_parameters.json: %v", err))
 		}
 		params := make([]worldgen.BiomeParameter, 0, len(raw.Biomes))
+		seen := make(map[string]bool)
 		for _, e := range raw.Biomes {
 			dp, ok := depthRange(e.Param.Depth)
 			if !ok {
 				continue // malformed depth; skip defensively
 			}
 			params = append(params, makeBiomeParameter(e, dp))
+			if !seen[e.Biome] {
+				seen[e.Biome] = true
+				biomeOrder = append(biomeOrder, e.Biome)
+			}
 		}
 		biomeTable = worldgen.NewParameterTable(params)
 	})
 	return biomeTable
 }
 
+func possibleBiomeOrder() []string {
+	loadBiomeTable()
+	return biomeOrder
+}
+
 // makeBiomeParameter converts a raw JSON entry into a BiomeParameter, mapping
 // the [min,max] ranges to quantized ClimateRanges. depth is a ClimateRange
 // (exact range for scalar depths, explicit range for cave biomes).

internal/world/feature_sorter_test.go

@@ -0,0 +1,28 @@
+package world
+
+import (
+	"slices"
+	"testing"
+
+	"regionio/internal/worldgen"
+)
+
+func TestOverworldFeatureStepsBuildWithoutCycles(t *testing.T) {
+	set, err := worldgen.LoadFeatureSet()
+	if err != nil {
+		t.Fatal(err)
+	}
+	steps, err := set.FeatureSteps(possibleBiomeOrder())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(steps) != 11 {
+		t.Fatalf("feature steps = %d, want 11", len(steps))
+	}
+	ores := steps[undergroundOresStage]
+	for _, name := range []string{"minecraft:ore_clay", "minecraft:ore_diamond", "minecraft:ore_tuff"} {
+		if !slices.Contains(ores, name) {
+			t.Errorf("underground ores missing %s", name)
+		}
+	}
+}