New pointer-event handling for full-screen terminal UI panes

What

• The terminal input handler now dispatches onPointerHover, onPointerDrag, onPointerPress and onPointerRelease callbacks, in addition to existing selection/click handling, and walks the UI's node tree to find and invoke a node's own pointer handler when in alt-screen (full-screen) mode.
• The internal terminal app context gained a subscribePointerPresses subscription, alongside the existing subscribeLayout. The renderer implements it with a real listener set and a tellPressedNowhere() method that fires listeners with null; this is used by a consumer to deselect focus when a click lands elsewhere.

Why This lays groundwork for interactive full-screen (alt-screen) panes to handle mouse events like hovering, dragging and clicking directly, and lets UI components react when a click happens outside them, such as clearing focus.