Expose native filesystem sources through public loading API

lifecycle.go

@@ -28,10 +28,14 @@ type Options[T any] struct {
 	Overrides  []Override
 }
 
-// FilesystemContext supplies filesystem roots for imports and overlays.
+// FilesystemContext supplies roots for imports and overlays. With FS nil,
+// BaseDir and Root use absolute host paths. With FS set, they use slash-relative
+// paths in that namespace ("." denotes its root). File sources derive BaseDir
+// from the selected file; a native file's explicit Root limits its scope.
 type FilesystemContext struct {
 	BaseDir string
 	Root    string
+	FS      *Filesystem
 }
 
 // Override describes one explicitly supplied application override. Value uses
@@ -223,6 +227,10 @@ func toEngineOptions[T any](options Options[T]) engine.Options[T] {
 	}
 	if options.Filesystem != nil {
 		result.Filesystem = &engine.FilesystemContext{BaseDir: options.Filesystem.BaseDir, Root: options.Filesystem.Root}
+		if options.Filesystem.FS != nil {
+			filesystem := options.Filesystem.FS.value
+			result.Filesystem.FS = &filesystem
+		}
 	}
 	if options.Overrides != nil {
 		result.Overrides = make([]engine.Override, len(options.Overrides))