docs: document the rewritten event system The entry still described the removed design: base-class events with category bitfields and AsyncEventBus. Replace it with the architecture that ships: the Event concept, sequential type IDs, the std::variant input path through the LayerStack, and the EventBus pub/sub path (immediate + double-buffered). Add a std::formatter how-to for user events and a migration table. closes #33
Started writing about the new Event System
docs: add comprehensive architecture and systems documentation to the wiki