[Architecture] Design plugin architecture and SPI for storage backends and query extensions #618

Summary

Design and formalize the Service Provider Interface (SPI) and plugin architecture for corese-next, defining extension points for future storage backends and custom execution hooks.

Objectives and Technical Scope

1. Storage Backend SPI

• Formalize public contracts decoupling the core query engine from concrete storage backends.
• Specify interfaces required for future persistent on-disk implementations and remote federation.

2. Extension Points and Lifecycle

• Define extension points for custom built-in functions, custom rule delegates, and query listeners.
• Establish service discovery mechanisms (Java ServiceLoader / SPI) with deterministic registration.

Acceptance Criteria

[ ] SPI contracts and public interfaces are clearly documented in docs/architecture/plugin-spi.md.
[ ] Contract interfaces prevent implementation leaks into public APIs.
[ ] A prototype proof-of-concept custom plugin demonstrates loading through the new SPI.