events: add Manager event system for inter-entity signaling

[Akif-jpg]

• Add new package engine/systems/events with:
  • Manager, Channel, Subscription, EventData types
  • Async (Emit) and sync (EmitSync) delivery methods
  • Thread-safe subscription management using RWMutex
  • Utilities: CreateChannel, GetChannel, RemoveChannel, Clear
• Add unit tests covering basic behavior, concurrency, types and benchmarks (src/engine/systems/events/event_manager_test.go)
• Add examples demonstrating entity communication, host integration and channel management (src/engine/systems/events/event_manager_example_test.go). Examples use EmitSync where deterministic output is required.
• Integrate event manager into Host:
  • add eventManager *events.Manager field
  • initialize in NewHost and expose via EventManager() getter
  • clear manager during Teardown
• All new comments are in English and follow project style.

This commit implements a publish-subscribe event system intended for lightweight, in-process signaling between entities and systems.

[BrentFarris]

Sorry about the delayed response here, I've not ignored or forgotten about this pull request. I've just been a bit tied up in some other features I've been working on. Thank you for the PR!

Adding in a pub/sub is a bit of an architecture change for things like entity destruction, but it does simplify a lot of other types of events. I can see it being useful as a framework tool for game developers. I'll try and make some time to think more about something like this being in architecture or as a helper for developers.