Correct mounted mapping diagnostic paths

internal/engine/load_input.go

@@ -29,6 +29,15 @@ func assembleBaseInputTracked[T any](source Source, options Options[T], destinat
 	if len(diagnostics) != 0 {
 		input.sources = sourceInfos(parsed.source)
 		setSourceDestinations(input.sources, destination)
+		// Mapping value issues already identify application content. Other early
+		// failures describe source syntax or access, not a placed value.
+		if source.kind == sourceMapping && source.mapIssue != nil && len(destination.segments) != 0 {
+			for index := range diagnostics {
+				relative := diagnostics[index].Path
+				segments := clonePathSegments(destination.segments, len(relative.segments))
+				diagnostics[index].Path = Path{segments: append(segments, relative.segments...)}
+			}
+		}
 		return input, diagnostics
 	}
 	if options.Imports {