TextUI 0.8.0

What's new

• Add public textui-layout-widget and textui-attach-widget APIs for embedding multiline widgets in ordinary buffers.
• Add top-level width-aware block widgets through :textui-layout and :textui-attach.
• Validate attachment bounds, text preservation, and widget lifecycle contracts.
• Preserve the existing single-line native widget behavior and stable location ordering.

Verification

• 140 ERT tests pass.
• Byte compilation and whitespace checks pass.