Export and display Remote Braille through the reader

A controlled reader renders accepted application frames in a new
virtual-only cthulhu-braille mode at the controller's width and applies
its pan and route gestures; reader UI, flashes and secure frames are
never exported, and hidden frames are unroutable. A controller reader
reports its display width, shows peer cells as an untranslated raw
frame and forwards local pan and routing keys. The physical helper now
reports its width and edge pans.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/app/braille_worker.go

@@ -45,6 +45,14 @@ type brailleWorker struct {
 	wake            chan struct{}
 	cancel          context.CancelFunc
 	done            chan struct{}
+	// Remote Braille. cells is the connected display width (0 unknown).
+	// remoteExport receives accepted application frames; remoteFrame shows a
+	// peer's cells in place of them; remoteGesture receives display keys
+	// pressed while remoteFrame is shown. All are guarded by mu.
+	cells         int
+	remoteExport  func(braille.Frame)
+	remoteFrame   *braille.Frame
+	remoteGesture func(gesture string, index int)
 }
 
 func newBrailleWorker(logger *slog.Logger) (*brailleWorker, error) {