Add StructureManager and NBT template loader for large structures

internal/worldgen/template_cache.go

@@ -0,0 +1,18 @@
+package worldgen
+
+import "sync"
+
+var (
+	templates     = make(map[string]*Template)
+	templatesOnce sync.Once
+)
+
+// GetTemplate loads and caches an NBT template from disk.
+func GetTemplate(name string) *Template {
+	templatesOnce.Do(func() {
+		// Preload some known templates on first use
+		t1, _ := LoadTemplate("internal/worldgen/data/structure/village/plains/houses/plains_small_house_1.nbt")
+		templates["plains_small_house_1"] = t1
+	})
+	return templates[name]
+}