feat: add De Bruijn indexed reduction engine

Add a new interpreter option (-i debruijn) that uses De Bruijn indices
for variable representation, eliminating the need for variable renaming
during substitution.

- Add -i flag to select interpreter (lambda or debruijn)
- Create debruijn package with Expression types (Variable with index,
  Abstraction without parameter, Application)
- Implement shift and substitute operations for De Bruijn indices
- Add conversion functions between lambda and De Bruijn representations
- Update CLI to support switching between interpreters
- Add De Bruijn tests to verify all samples pass

Closes #26

pkg/debruijn/expression.go

@@ -0,0 +1,119 @@
+// Package debruijn provides De Bruijn indexed lambda calculus expressions.
+// De Bruijn indices eliminate the need for variable names by using numeric
+// indices to refer to bound variables, avoiding capture issues during substitution.
+package debruijn
+
+import "git.maximhutz.com/max/lambda/pkg/expr"
+
+// Expression is the interface for all De Bruijn indexed expression types.
+// It embeds the general expr.Expression interface for cross-mode compatibility.
+type Expression interface {
+	expr.Expression
+	Accept(Visitor)
+}
+
+/** ------------------------------------------------------------------------- */
+
+// Abstraction represents a lambda abstraction without a named parameter.
+// In De Bruijn notation, the parameter is implicit and referenced by index 0
+// within the body.
+type Abstraction struct {
+	body Expression
+}
+
+// Body returns the body of the abstraction.
+func (a *Abstraction) Body() Expression {
+	return a.body
+}
+
+// Accept implements the Visitor pattern.
+func (a *Abstraction) Accept(v Visitor) {
+	v.VisitAbstraction(a)
+}
+
+// String returns the De Bruijn notation string representation.
+func (a *Abstraction) String() string {
+	return Stringify(a)
+}
+
+// NewAbstraction creates a new De Bruijn abstraction with the given body.
+func NewAbstraction(body Expression) *Abstraction {
+	return &Abstraction{body: body}
+}
+
+/** ------------------------------------------------------------------------- */
+
+// Application represents the application of one expression to another.
+type Application struct {
+	abstraction Expression
+	argument    Expression
+}
+
+// Abstraction returns the function expression being applied.
+func (a *Application) Abstraction() Expression {
+	return a.abstraction
+}
+
+// Argument returns the argument expression.
+func (a *Application) Argument() Expression {
+	return a.argument
+}
+
+// Accept implements the Visitor pattern.
+func (a *Application) Accept(v Visitor) {
+	v.VisitApplication(a)
+}
+
+// String returns the De Bruijn notation string representation.
+func (a *Application) String() string {
+	return Stringify(a)
+}
+
+// NewApplication creates a new application expression.
+func NewApplication(abstraction Expression, argument Expression) *Application {
+	return &Application{abstraction: abstraction, argument: argument}
+}
+
+/** ------------------------------------------------------------------------- */
+
+// Variable represents a De Bruijn indexed variable.
+// The index indicates how many binders to skip to find the binding abstraction.
+// The label is an optional hint for display purposes.
+type Variable struct {
+	index int
+	label string
+}
+
+// Index returns the De Bruijn index.
+func (v *Variable) Index() int {
+	return v.index
+}
+
+// Label returns the optional variable label.
+func (v *Variable) Label() string {
+	return v.label
+}
+
+// Accept implements the Visitor pattern.
+func (v *Variable) Accept(visitor Visitor) {
+	visitor.VisitVariable(v)
+}
+
+// String returns the De Bruijn notation string representation.
+func (v *Variable) String() string {
+	return Stringify(v)
+}
+
+// NewVariable creates a new De Bruijn variable with the given index and label.
+func NewVariable(index int, label string) *Variable {
+	return &Variable{index: index, label: label}
+}
+
+/** ------------------------------------------------------------------------- */
+
+// Visitor interface for traversing De Bruijn expressions.
+type Visitor interface {
+	VisitAbstraction(*Abstraction)
+	VisitApplication(*Application)
+	VisitVariable(*Variable)
+}