D1: Plugin system architecture doc #161

Summary

Write the plugin system architecture documentation covering DLL interface, engine API table, module lifecycle, and event system.

Status: COMPLETE

Document created and merged to main:

• docs/architecture/plugin-system.md

Covers: DLL interface, `obc_module_load` entry point, module lifecycle (load → run → shutdown), stock module split (9 modules), API versioning, configuration.