Document and benchmark sibling inheritance

lifecycle.go

@@ -79,8 +79,9 @@ type Result[T any] struct {
 }
 
 // Load parses and strictly decodes zero or one selected source, explicitly enabled
-// imports, and one selected overlay. It accepts at most one Options value and
-// returns no result when any error is reported.
+// imports, and one selected overlay. After source precedence, built-in sibling
+// `extends` relationships resolve before defaults and typed decoding. It accepts
+// at most one Options value and returns no result when any error is reported.
 func Load[T any](source Source, supplied ...Options[T]) (*Result[T], error) {
 	options := make([]engine.Options[T], len(supplied))
 	for index := range supplied {