Activate sibling inheritance in configuration loading

metadata.go

@@ -13,8 +13,8 @@ func (m Metadata) Lookup(path Path) (FieldInfo, bool) {
 	return fromEngineFieldInfo(value), ok
 }
 
-// IsExplicit reports whether a surviving input contribution explicitly defined
-// path.
+// IsExplicit reports whether a surviving input contribution, including one
+// inherited from a sibling, explicitly defined path.
 func (m Metadata) IsExplicit(path Path) bool {
 	return m.value.IsExplicit(toEnginePath(path))
 }