Potential interoperability with A2UI

[liady]

Exploring potential interoperability directions with A2UI.

Two directions -

1. MCP Apps content being rendered as a "black box" A2UI leaf component, or -
2. Abstract "A2UI" content returned from an MCP Apps server (instead of an HTML).
   Both ways make sense, and the two methods (MCP-driven vs Agent driven) can be complementing this way.

There's a constant tension between the hosts' needs for UI control and streamlining, and the servers' needs for branding and customized UI. Each method puts emphasis on a different end of this scale.