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/config/config.go

@@ -9,4 +9,5 @@ type Config struct {
 	Explanation bool        // Whether or not to print an explanation of the reduction.
 	Profile     string      // If not nil, print a CPU profile during execution.
 	Statistics  bool        // Whether or not to print statistics.
+	Interpreter string      // The interpreter to use: "lambda" or "debruijn".
 }

internal/config/parse_from_args.go

@@ -14,6 +14,7 @@ func FromArgs() (*Config, error) {
 	profile := flag.String("p", "", "CPU profiling. If an output file is defined, the program will profile its execution and dump its results into it.")
 	file := flag.String("f", "", "File. If set, read source from the specified file.")
 	output := flag.String("o", "", "Output. If set, write result to the specified file. Use '-' for stdout (default).")
+	interpreter := flag.String("i", "lambda", "Interpreter. Choose 'lambda' or 'debruijn' reduction engine (default: lambda).")
 	flag.Parse()
 
 	// Parse source type.
@@ -45,6 +46,11 @@ func FromArgs() (*Config, error) {
 		destination = FileDestination{Path: *output}
 	}
 
+	// Validate interpreter flag.
+	if *interpreter != "lambda" && *interpreter != "debruijn" {
+		return nil, fmt.Errorf("invalid interpreter: %s (must be 'lambda' or 'debruijn')", *interpreter)
+	}
+
 	return &Config{
 		Source:      source,
 		Destination: destination,
@@ -52,5 +58,6 @@ func FromArgs() (*Config, error) {
 		Explanation: *explanation,
 		Profile:     *profile,
 		Statistics:  *statistics,
+		Interpreter: *interpreter,
 	}, nil
 }