[FEATURE]: Add a public pure-Dart adapter for Flutter catalogs and functions

- [x] I have searched the existing issues to make sure this feature has not already been requested.

## Is your feature request related to a problem? Please describe.

Flutter applications define catalogs/functions through Flutter GenUI APIs, while pure-Dart tooling uses a2ui_core types. No supported adapter proves which Flutter behavior is representable headlessly. Copying JSON Schema alone misses references, structural slots, bindings, validation, functions, and renderer fallbacks.

## Describe the Proposed Solution

Provide a public pure-Dart adaptation API that consumes serializable/public Flutter catalog definitions and produces a resolved a2ui_core catalog, explicit unsupported-schema or unresolved-$ref diagnostics, structural/dynamic/action metadata, a registry of caller-supplied pure deterministic function stubs, and a compatibility report distinguishing adapted, custom-driver-required, and unsupported components. Fail closed; do not serialize widget builders or arbitrary closures.

## Describe Alternatives Considered

Manual redeclaration drifts from the catalog shown to the agent. Flutter widget reflection is not portable. Treating schema conversion as behavior parity is unsafe.

## Additional Context

#1528 provides schema-driven binding/reference-resolution groundwork and #1282 provides relevant Node Layer work. This request focuses on the public Dart/Flutter adaptation boundary.