fix: update main and tests for new Reducer API

Update call sites to match the new Reducer interface where the expression
is passed to the constructor and Reduce() takes no arguments.

pkg/reducer/reducer.go

@@ -20,7 +20,7 @@ type Reducer interface {
 	// Emits StartEvent before reduction, StepEvent after each step, and
 	// StopEvent after completion.
 	// Returns the final reduced expression.
-	Reduce(e expr.Expression) expr.Expression
+	Reduce()
 
 	// Expression returns the current expression state.
 	Expression() expr.Expression