Rename internal `SurfaceRegistry` events to resolve name clashes in package exports

[yjbanov]

Context

Currently in genui.dart:L16-17, we have to hide two classes in our exports due to name clashes:

export 'src/engine.dart' hide SurfaceAdded, SurfaceRemoved;

The classes are defined in two places:

1. SurfaceUpdate events (Public UI-level updates for facades/consumers):
   • Location: ui_models.dart:L402-438
   • Classes: SurfaceAdded, ComponentsUpdated, and SurfaceRemoved (subclasses of SurfaceUpdate).
2. RegistryEvent events (Internal engine-level registry updates):
   • Location: surface_registry.dart:L12-36
   • Classes: SurfaceAdded, SurfaceRemoved, and SurfaceUpdated (subclasses of RegistryEvent).

Proposed Solution

To make the API clean, robust, and unambiguous without needing hide clauses, we should prefix the event classes in surface_registry.dart to associate them explicitly with the registry.

1. Rename classes

In surface_registry.dart:

• RegistryEvent $\rightarrow$ SurfaceRegistryEvent
• SurfaceAdded $\rightarrow$ SurfaceRegistryAdded
• SurfaceRemoved $\rightarrow$ SurfaceRegistryRemoved
• SurfaceUpdated $\rightarrow$ SurfaceRegistryUpdated

2. Update surface controller

In surface_controller.dart
Update the mapping logic in surfaceUpdates (around L57-67) to match the new class names:

  @override
  Stream<SurfaceUpdate> get surfaceUpdates => _registry.events.map(
    (e) => switch (e) {
      surface_reg.SurfaceRegistryAdded(:final surfaceId, :final definition) =>
        SurfaceAdded(surfaceId, definition),
      surface_reg.SurfaceRegistryUpdated(:final surfaceId, :final definition) =>
        ComponentsUpdated(surfaceId, definition),
      surface_reg.SurfaceRegistryRemoved(:final surfaceId) => 
        SurfaceRemoved(surfaceId),
    },
  );

Benefits

• Cleaner Exports: No need to hide classes in the main package entrypoint.
• Unambiguous Code: Developers importing internal libraries or both files directly won't run into naming collisions.
• Stronger Semantics: Clearly distinguishes between engine-internal registry state events (SurfaceRegistryAdded) and public UI-level state events (SurfaceAdded).