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/shift.go

@@ -0,0 +1,32 @@
+package debruijn
+
+// Shift increments all free variable indices in an expression by the given amount.
+// A variable is free if its index is >= the cutoff (depth of nested abstractions).
+// This is necessary when substituting an expression into a different binding context.
+func Shift(expr Expression, amount int, cutoff int) Expression {
+	switch e := expr.(type) {
+	case *Variable:
+		if e.index >= cutoff {
+			return NewVariable(e.index+amount, e.label)
+		}
+		return e
+
+	case *Abstraction:
+		newBody := Shift(e.body, amount, cutoff+1)
+		if newBody == e.body {
+			return e
+		}
+		return NewAbstraction(newBody)
+
+	case *Application:
+		newAbs := Shift(e.abstraction, amount, cutoff)
+		newArg := Shift(e.argument, amount, cutoff)
+		if newAbs == e.abstraction && newArg == e.argument {
+			return e
+		}
+		return NewApplication(newAbs, newArg)
+
+	default:
+		return expr
+	}
+}