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>

pkg/debruijn/substitute.go

@@ -0,0 +1,68 @@
+package debruijn
+
+// Shift increments all free variable indices by delta when crossing depth abstractions.
+func Shift(expr Expression, delta int, depth int) Expression {
+	switch e := expr.(type) {
+	case *Variable:
+		if e.index >= depth {
+			return NewVariable(e.index+delta, e.label)
+		}
+		return e
+
+	case *Abstraction:
+		newBody := Shift(e.body, delta, depth+1)
+		if newBody == e.body {
+			return e
+		}
+		return NewAbstraction(newBody)
+
+	case *Application:
+		newAbs := Shift(e.abstraction, delta, depth)
+		newArg := Shift(e.argument, delta, depth)
+		if newAbs == e.abstraction && newArg == e.argument {
+			return e
+		}
+		return NewApplication(newAbs, newArg)
+
+	default:
+		return expr
+	}
+}
+
+// Substitute replaces variable at index 0 with replacement in expr.
+// This assumes expr is the body of an abstraction being applied.
+func Substitute(expr Expression, replacement Expression) Expression {
+	return substitute(expr, 0, replacement)
+}
+
+// substitute replaces variable at targetIndex with replacement, adjusting indices as needed.
+func substitute(expr Expression, targetIndex int, replacement Expression) Expression {
+	switch e := expr.(type) {
+	case *Variable:
+		if e.index == targetIndex {
+			return Shift(replacement, targetIndex, 0)
+		}
+		if e.index > targetIndex {
+			return NewVariable(e.index-1, e.label)
+		}
+		return e
+
+	case *Abstraction:
+		newBody := substitute(e.body, targetIndex+1, replacement)
+		if newBody == e.body {
+			return e
+		}
+		return NewAbstraction(newBody)
+
+	case *Application:
+		newAbs := substitute(e.abstraction, targetIndex, replacement)
+		newArg := substitute(e.argument, targetIndex, replacement)
+		if newAbs == e.abstraction && newArg == e.argument {
+			return e
+		}
+		return NewApplication(newAbs, newArg)
+
+	default:
+		return expr
+	}
+}