Schedule source chunk features globally

internal/world/feature_scheduler.go

@@ -0,0 +1,21 @@
+package world
+
+// decorationSource is a source chunk whose feature pass may inspect or write a
+// target chunk. Vanilla FEATURES has a one-chunk block-state write radius.
+type decorationSource struct {
+	X, Z int32
+}
+
+// decorationSources returns source chunks in deterministic X-major/Z-minor
+// order. Replaying all nine against one mutable 3x3 terrain region makes target
+// output independent of cache request order while preserving each source's own
+// decoration seed and placement origin.
+func decorationSources(targetX, targetZ int32) []decorationSource {
+	sources := make([]decorationSource, 0, 9)
+	for sourceX := targetX - 1; sourceX <= targetX+1; sourceX++ {
+		for sourceZ := targetZ - 1; sourceZ <= targetZ+1; sourceZ++ {
+			sources = append(sources, decorationSource{X: sourceX, Z: sourceZ})
+		}
+	}
+	return sources
+}