feat(common,core): Add ReducerAction.Skip and rename ReadModelAction to ProjectionAction

[javiertoledo]

Problem

1. Entity reducers have no way to signal that an event should be ignored/skipped. When receiving an unexpected event (e.g., an update event for an entity that doesn't exist), reducers must either return an invalid/placeholder entity or throw an exception (which goes against the principle that reducers should be pure).

2. The existing ReadModelAction enum uses non-semantic names (Nothing instead of Skip).

3. Naming is inconsistent between concepts - projections have ReadModelAction but reducers have no equivalent.

Proposed Solution

Create a consistent, semantic naming pattern across both reducers and projections:

// For Projections (replace ReadModelAction)
export enum ProjectionAction {
  Skip,    // was ReadModelAction.Nothing
  Delete,  // was ReadModelAction.Delete
}
export type ProjectionResult<TReadModel> = TReadModel | ProjectionAction

// For Reducers (new)
export enum ReducerAction {
  Skip,
}
export type ReducerResult<TEntity> = TEntity | ReducerAction

Example Usage

Entity Reducer

import { ReducerAction, ReducerResult } from '@magek/common'

@Entity
export class Product {
  constructor(readonly id: UUID, readonly name: string) {}

  @Reduces(ProductUpdated)
  public static reduceProductUpdated(
    event: ProductUpdated,
    current?: Product
  ): ReducerResult<Product> {
    if (!current) {
      // Can't update a non-existent product - skip this event
      return ReducerAction.Skip
    }
    return new Product(current.id, event.newName)
  }
}

Read Model Projection

import { ProjectionAction, ProjectionResult } from '@magek/common'

@ReadModel
export class UserReadModel {
  @Projects(User, 'id')
  public static projectUser(
    entity: User,
    current?: UserReadModel
  ): ProjectionResult<UserReadModel> {
    if (entity.deleted) {
      return ProjectionAction.Delete
    }
    if (!hasRelevantChanges(entity, current)) {
      return ProjectionAction.Skip
    }
    return new UserReadModel(entity.id, entity.name)
  }
}

Files to Modify

File	Changes
packages/common/src/concepts/reducer-metadata.ts	Add ReducerAction, ReducerResult
packages/common/src/concepts/projection-metadata.ts	Replace ReadModelAction with ProjectionAction
packages/common/src/index.ts	Update exports
packages/core/src/services/event-store.ts	Handle ReducerAction.Skip
packages/core/src/services/read-model-store.ts	Use ProjectionAction
docs/content/architecture/entity.md	Document ReducerAction.Skip
docs/content/architecture/read-model.md	Use ProjectionAction
packages/core/test/services/event-store.test.ts	Add reducer skip tests
packages/core/test/services/read-model-store.test.ts	Use ProjectionAction

Summary

Concept	Skip	Delete
Entity Reducer	ReducerAction.Skip	N/A (events are immutable)
Read Model Projection	ProjectionAction.Skip	ProjectionAction.Delete