Ship typed Surface contracts and the open-source Flutter shell

[LeftTwixWand]

Outcome

Provide portable typed Surface contracts and a minimal open-source Flutter shell that renders them consistently.

Acceptance criteria

• Support standard layouts, text, buttons, forms, tables, pages, navigation, validation, loading states, and error states.
• Define typed actions.
• Enforce single-writer ownership.
• Support slots and contributions.
• Require a portable fallback for every Surface.
• Pass adapter contract tests.
• Keep native bootstrap code minimal.

Non-goals

• Building feature logic into the shell or requiring platform-specific renderers.