Document and benchmark sibling inheritance

layered.go

@@ -30,8 +30,9 @@ type LayeredOptions[T any] struct {
 // imports with each input's tier. It strictly composes contributions within
 // each underlay, then recursively combines underlays in listed order beneath
 // main, a selected overlay, and CLI overrides. Defaults, decoding, resolution,
-// and validation run once on the assembled input. Source IDs follow acquisition
-// order, not precedence order. Zero or one options value is accepted; any error
+// and validation run once after sibling `extends` expansion of the assembled
+// input. Source IDs follow acquisition order, not precedence order. Zero or
+// one options value is accepted; any error
 // returns a nil result.
 func LoadLayered[T any](main Input, supplied ...LayeredOptions[T]) (*Result[T], error) {
 	options := make([]engine.LayeredOptions[T], len(supplied))