Refactors the event emitter system from string-based messages to a
type-safe generic implementation using typed events. Consolidates
separate tracker packages into a unified plugins architecture.
Changes:
- Replace Emitter with BaseEmitter[E comparable] using generics
- Add Event type with StartEvent, StepEvent, and StopEvent constants
- Create Listener[E] interface with BaseListener implementation
- Consolidate explanation, performance, and statistics trackers into
internal/plugins package
- Simplify main CLI by using plugin constructors instead of manual
event subscription
- Add Items() iterator method to Set for idiomatic range loops
