v2: Events DSL - Add event() function and update EventMetadata

[javiertoledo]

Parent

Part of #739

Context

Events currently use class-based metadata:

interface EventMetadata {
  readonly class: Class<EventInterface>
}

Events need to be migrated to the data-oriented pattern established by commands.

Proposed Changes

1. Create event() DSL function in packages/core/src/dsl/event.ts
2. Update EventMetadata to use name + entityID function instead of class reference
3. Update event dispatching/handling to use new metadata
4. Reimplement @Event decorator as thin wrapper around DSL
5. Update reducers that reference events

Design Considerations

• Events are simpler than commands (no authorization, no hooks)
• Events have an entityID() method that links them to entities — this needs a clean DSL representation
• Reducers reference events by name — ensure compatibility
• Event handlers receive event instances — consider how DSL events work with handlers

Acceptance Criteria

• event() DSL function created
• EventMetadata updated to data-oriented format
• @Event decorator reimplemented as wrapper
• All existing tests pass
• New tests for DSL event definition