constructor() {
      this.events = /* @__PURE__ */ new Map();
    }
Registers a new event listener for the specified event.
- Creates new Set for event if it doesn't exist
- Adds callback to the Set of listeners
- Returns unsubscribe function for cleanup

@param {string} eventName - Event identifier
@param {Function} callback - Event handler function
@returns {Function} Unsubscribe function