Integrate private destinations for layered inputs

internal/engine/destination.go

@@ -29,6 +29,20 @@ func normalizeDestination(path Path) (Path, error) {
 	return Path{segments: segments}, nil
 }
 
+func invalidDestinationDiagnostic(destination Path) Diagnostic {
+	return Diagnostic{
+		Code: CodeOptionsInvalid, Severity: SeverityError,
+		Path:    destination.clone(),
+		Message: "input destination must contain only non-reserved mapping keys",
+	}
+}
+
+func setSourceDestinations(sources []SourceInfo, destination Path) {
+	for index := range sources {
+		sources[index].Destination = destination.clone()
+	}
+}
+
 // placeDocument checks all per-document bounds before allocating destination
 // wrappers. Aggregate accounting remains the caller's responsibility.
 func placeDocument(body *document.Node, destination Path, sourceRoot document.Location, maxDepth, maxNodes int) (Path, *document.Node, error) {