Implement layered public API design (ADR-030)

[jwilger]

Summary

Implement the layered crate public API design established in ADR-030 and documented in ARCHITECTURE.md.

Background

The eventcore crate should export a minimal API for application developers, while eventcore-types provides the full API for backend implementers. This cleanup reduces API surface area, clarifies audience separation, and makes future evolution easier.

Acceptance Criteria

• EventCollector projector implemented in eventcore-testing (Implement EventCollector projector in eventcore-testing (ADR-031) #278)
• eventcore-examples crate created (Create eventcore-examples crate (ADR-032) #279)
• Integration tests migrated to eventcore-examples (Migrate integration tests to eventcore-examples #280)
• eventcore/src/lib.rs exports only application-developer types (Remove unnecessary exports from eventcore public API (ADR-030) #281)
• Internal types made pub(crate)
• Dead code removed
• All tests pass after API cleanup

Sub-Issues

• Implement EventCollector projector in eventcore-testing (ADR-031) #278 - Implement EventCollector projector in eventcore-testing (ADR-031)
• Create eventcore-examples crate (ADR-032) #279 - Create eventcore-examples crate (ADR-032)
• Migrate integration tests to eventcore-examples #280 - Migrate integration tests to eventcore-examples
• Remove unnecessary exports from eventcore public API (ADR-030) #281 - Remove unnecessary exports from eventcore public API

Related

• Architect continuous polling mode for run_projection API #276 - Architect continuous polling mode (future work)
• ADR-030: Layered Crate Public API Design
• ADR-031: Black-Box Integration Testing via Projections
• ADR-032: Integration Test Crate for End-to-End Testing

References

• ARCHITECTURE.md: Crate Organization section