Replaces the single-slot status-line flash with a top-right toast
stack over the focused pane. flashError, flashTransient, and
notifyAttention all push onto the same stack (cap 5, FIFO drop).
Ctrl-N dismisses the most recent toast; empty stack falls through to
the focused PTY so readline / nano / emacs / opencode bindings keep
working. A new "Clear notifications" palette item empties the stack.
