Add a declarative widget system that lets extensions place persistent
content above or below the input area. Widgets survive across agent
turns and are updated via ctx.SetWidget/ctx.RemoveWidget from any
event handler.
All types are concrete structs (Yaegi-safe, no interfaces cross the
interpreter boundary). Widget state lives on the Runner with mutex
protection, and WidgetUpdateEvent triggers BubbleTea re-renders.
Ed Zynda