Wait quietly for an absent braille display

Helper retries BrlAPI open in-process until BRLTTY answers, supervisor waits with the parked helper instead of kill/respawn cycles, exponential backoff and warn-once for real helper exits, bounded give-up so permanent failures still surface.

internal/app/braille_worker.go

@@ -31,6 +31,7 @@ type brailleWorker struct {
 	connected       bool
 	logger          *slog.Logger
 	path            string
+	readyGiveUp     time.Duration // Wedged-helper recycle bound; zero uses the default.
 	mu              sync.Mutex
 	frame           braille.Frame
 	baseFrame       braille.Frame