Answer architecture round 11: a source states its mount in a field its manifest, its labels and its file reads share, so the mount can come before or after the manifest; the README says a stand-in for the places is a record with a format

internal/datafiles/manifest.go

@@ -35,7 +35,7 @@ func (s Source) Manifest() (Manifest, error) {
 	if err := s.check(); err != nil {
 		return Manifest{}, err
 	}
-	b, err := fs.ReadFile(s.fsys, ManifestFile)
+	b, err := fs.ReadFile(s.fsys, path.Join(s.at, ManifestFile))
 	if errors.Is(err, fs.ErrNotExist) {
 		return Manifest{}, nil
 	}
@@ -50,12 +50,7 @@ func (s Source) Manifest() (Manifest, error) {
 }
 
 // ManifestPath is the manifest's path as errors name it.
-func (s Source) ManifestPath() string {
-	if m, isMounted := s.fsys.(mounted); isMounted {
-		return s.labelled(path.Join(m.at, ManifestFile))
-	}
-	return s.labelled(ManifestFile)
-}
+func (s Source) ManifestPath() string { return s.labelled(path.Join(s.at, ManifestFile)) }
 
 // decodeManifest decodes a manifest, refusing any key or shape it does not define, and
 // reports every mistake: first each entry's own, in path order, then each parent naming