feat: add De Bruijn index reduction engine

Closes #26

- Added -i flag to select interpreter (lambda or debruijn)
- Created debruijn package with Expression interface
  - Variable contains index and optional label
  - Abstraction contains only body (no parameter)
  - Application structure remains similar
- Implemented De Bruijn reduction without variable renaming
  - Shift operation handles index adjustments
  - Substitute replaces by index instead of name
- Abstracted Engine into interface with two implementations
  - LambdaEngine: original named variable engine
  - DeBruijnEngine: new index-based engine
- Added conversion functions between representations
  - LambdaToDeBruijn: converts named to indexed
  - DeBruijnToLambda: converts indexed back to named
  - SaccharineToDeBruijn: direct saccharine to De Bruijn
- Updated main to switch engines based on -i flag
- All test samples pass with both engines

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/engine/interface.go

@@ -0,0 +1,24 @@
+// Package "engine" provides an extensible interface for users to interfact with
+// λ-calculus.
+package engine
+
+import (
+	"git.maximhutz.com/max/lambda/internal/config"
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+)
+
+// Engine is an interface for reduction engines.
+type Engine interface {
+	Run()
+	GetResult() string
+	On(message string, fn func()) *emitter.Observer
+	Emit(message string)
+}
+
+// New creates the appropriate engine based on the config.
+func New(cfg *config.Config, input interface{}) Engine {
+	if cfg.Interpreter == "debruijn" {
+		return NewDeBruijnEngine(cfg, input)
+	}
+	return NewLambdaEngine(cfg, input)
+}