Terminal layout groundwork: transcript overlay, prompt container and a hidden left gutter

What

Several internal changes to how the terminal screen is laid out:

• The main screen gains a transcriptOverlay slot that shows a stack of toasts (small pop-up notices) over the conversation transcript.
• The scrollable area and the prompt layout receive a new promptMounted value. The row showing the current turn and the panel of queued messages are now wrapped in a container that knows whether verbose mode is on.
• A new kind of dialog also shows the session-title bar.
• A fourth rate-limit listener withdraws automatic continuation when the rate limit resets. Before, there were three.
• The message list gains a 2-column left gutter (yte = 2), with extra indent for incoming or labelled replies and a leftMargin for the sticky prompt. The gutter depends on a check that always returns false in this build, so it is never shown. Before, those replies had a fixed 2-column spacer.

Why

Most of this is plumbing with no clear visible effect yet. Toasts can now appear over the transcript, and the gutter is a transcript layout change that is built but not switched on.