fix: initialize BaseEmitter in Engine constructor

The Engine struct embeds BaseEmitter but wasn't initializing it,
causing a nil map panic when emitting events. Now properly
initializes the BaseEmitter using emitter.New[Event]().

internal/engine/engine.go

@@ -17,7 +17,11 @@ type Engine struct {
 
 // Create a new engine, given an unreduced λ-expression.
 func New(config *config.Config, expression *lambda.Expression) *Engine {
-	return &Engine{Config: config, Expression: expression}
+	return &Engine{
+		Config:      config,
+		Expression:  expression,
+		BaseEmitter: *emitter.New[Event](),
+	}
 }
 
 // Begin the reduction process.