refactor: extract Reducer interface and update engine to use abstractions

Introduce reducer.Reducer interface and update the engine to use abstract
expr.Expression and reducer.Reducer types, enabling pluggable reduction
strategies.

- Add pkg/reducer/reducer.go with Reducer interface.
- Add pkg/lambda/reducer.go with NormalOrderReducer implementation.
- Update engine to accept expr.Expression and reducer.Reducer.
- Update plugins to use expr.Expression directly (no pointer dereference).
- Update main and tests to pass reducer to engine.

Closes #30

cmd/lambda/lambda.go

@@ -8,6 +8,7 @@ import (
 	"git.maximhutz.com/max/lambda/internal/engine"
 	"git.maximhutz.com/max/lambda/internal/plugins"
 	"git.maximhutz.com/max/lambda/pkg/convert"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 )
 
@@ -33,8 +34,9 @@ func main() {
 	compiled := convert.SaccharineToLambda(ast)
 	logger.Info("compiled λ expression", "tree", compiled.String())
 
-	// Create reduction engine.
-	process := engine.New(options, &compiled)
+	// Create reduction engine with normal order reducer.
+	reducer := lambda.NewNormalOrderReducer()
+	process := engine.New(options, compiled, reducer)
 
 	// If the user selected to track CPU performance, attach a profiler to the
 	// process.
@@ -61,7 +63,7 @@ func main() {
 	process.Run()
 
 	// Return the final reduced result.
-	result := (*process.Expression).String()
+	result := process.Expression.String()
 	err = options.Destination.Write(result)
 	cli.HandleError(err)
 }

cmd/lambda/lambda_test.go

@@ -41,11 +41,12 @@ func runSample(samplePath string) (string, error) {
 		Verbose:     false,
 	}
 
-	// Create and run the engine.
-	process := engine.New(cfg, &compiled)
+	// Create and run the engine with normal order reducer.
+	reducer := lambda.NewNormalOrderReducer()
+	process := engine.New(cfg, compiled, reducer)
 	process.Run()
 
-	return lambda.Stringify(compiled) + "\n", nil
+	return process.Expression.String() + "\n", nil
 }
 
 // Test that all samples produce expected output.