Event Sourcing Engine

# Event Sourcing Engine PRD

## Overview
Build a complete event sourcing system with an append-only event store, aggregate roots that rebuild state from event streams, materialized projections for read models, snapshot support for performance, and event versioning for schema evolution.

## Requirements
- Append-only event store with ordered events per aggregate
- Event schema: aggregateId, type, payload, metadata, timestamp, version number
- Optimistic concurrency control using expected version on append
- Per-aggregate event streams for loading a single aggregate's history
- Global event stream ordered across all aggregates for system-wide subscriptions
- Projections that subscribe to specific event types and build denormalized read models
- Projection replay: rebuild any projection from scratch by replaying all events
- Snapshot support: periodic aggregate state snapshots to avoid replaying full history
- Event versioning with upcasters that transform old event formats to current schema
- Event metadata: causation ID, correlation ID, and user ID for traceability
- Subscription API for real-time notification when new events are appended
- Catch-up subscription that starts from a specific stream position
- Idempotent event processing with deduplication to prevent double-processing
- Aggregate root base class that loads state from events and applies new events
- Persistence to disk so events survive process restarts
- CLI for store management: inspect events, replay projections, create snapshots
- Handle 100K+ events per aggregate stream

## Tech Stack
- TypeScript / Node.js
- No external event sourcing or CQRS libraries — raw implementation required
- File system for persistent event storage

## Scoring Criteria
- **Functional (40%)**: Events append and replay correctly, aggregates rebuild state, projections materialize
- **Quality (20%)**: Proper concurrency control, clean aggregate design, reliable persistence
- **Fidelity (25%)**: All features including snapshots, versioning, catch-up subscriptions, and deduplication
- **Speed (15%)**: Time bonus