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/debruijn_engine.go

@@ -0,0 +1,36 @@
+package engine
+
+import (
+	"git.maximhutz.com/max/lambda/internal/config"
+	"git.maximhutz.com/max/lambda/pkg/debruijn"
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+)
+
+// A process for reducing one λ-expression using De Bruijn indices.
+type DeBruijnEngine struct {
+	Config     *config.Config
+	Expression *debruijn.Expression
+	emitter.Emitter
+}
+
+// NewDeBruijnEngine creates a new De Bruijn engine.
+func NewDeBruijnEngine(config *config.Config, expression interface{}) *DeBruijnEngine {
+	expr := expression.(*debruijn.Expression)
+	return &DeBruijnEngine{Config: config, Expression: expr}
+}
+
+// Run begins the reduction process.
+func (e *DeBruijnEngine) Run() {
+	e.Emit("start")
+
+	debruijn.ReduceAll(e.Expression, func() {
+		e.Emit("step")
+	})
+
+	e.Emit("end")
+}
+
+// GetResult returns the stringified result.
+func (e *DeBruijnEngine) GetResult() string {
+	return debruijn.Stringify(*e.Expression)
+}