[Reactivity] Server-side fanout codegen in internal/appgen

[cssbruno]

Generate server-side fanout code for presentation-event driven reactivity.

Scope:

• Use contract metadata to connect presentation events to generated fanout hooks.
• Keep guard and ownership boundaries explicit.

Acceptance criteria:

• Generated app code exposes the required fanout registration path.
• Tests cover event delivery to the supported generated route/component surface.
• Unsupported event categories fail validation.