refactor: rename interpreter to runtime and use receiver methods (#39)

## Description

The codebase previously used "interpreter" terminology and standalone functions for expression operations.
This PR modernizes the architecture by renaming to "runtime" and converting operations to receiver methods.

- Rename `pkg/interpreter` to `pkg/runtime`.
- Move `ReduceOnce` to new `pkg/normalorder` package for reduction strategy isolation.
- Convert standalone functions (`Substitute`, `Rename`, `GetFree`, `IsFree`) to receiver methods on concrete expression types.
- Change `Set` from pointer receivers to value receivers for simpler usage.
- Update all references from "interpreter" to "runtime" terminology throughout the codebase.

### Decisions

- Operations like `Substitute`, `Rename`, `GetFree`, and `IsFree` are now methods on the `Expression` interface, implemented by each concrete type (`Variable`, `Abstraction`, `Application`).
- The `normalorder` package isolates the normal-order reduction strategy, allowing future reduction strategies to be added in separate packages.
- `Set` uses value receivers since Go maps are reference types and don't require pointer semantics.

## Benefits

- Cleaner API: `expr.Substitute(target, replacement)` instead of `Substitute(expr, target, replacement)`.
- Better separation of concerns: reduction strategies are isolated from expression types.
- Consistent terminology: "runtime" better reflects the execution model.
- Simpler `Set` usage without needing to manage pointers.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`). Always use underscores.
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).

Reviewed-on: #39
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

cmd/lambda/lambda.go

@@ -7,7 +7,7 @@ import (
 	"git.maximhutz.com/max/lambda/internal/config"
 	"git.maximhutz.com/max/lambda/internal/plugins"
 	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
+	"git.maximhutz.com/max/lambda/pkg/normalorder"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 )
 
@@ -34,34 +34,34 @@ func main() {
 	logger.Info("compiled λ expression", "tree", compiled.String())
 
 	// Create reducer with the compiled expression.
-	interpreter := lambda.NewInterpreter(compiled)
+	runtime := normalorder.NewRuntime(compiled)
 
 	// If the user selected to track CPU performance, attach a profiler.
 	if options.Profile != "" {
-		plugins.NewPerformance(options.Profile, interpreter)
+		plugins.NewPerformance(options.Profile, runtime)
 	}
 
 	// If the user selected to produce a step-by-step explanation, attach an
 	// observer.
 	if options.Explanation {
-		plugins.NewExplanation(interpreter)
+		plugins.NewExplanation(runtime)
 	}
 
 	// If the user opted to track statistics, attach a tracker.
 	if options.Statistics {
-		plugins.NewStatistics(interpreter)
+		plugins.NewStatistics(runtime)
 	}
 
 	// If the user selected for verbose debug logs, attach a reduction tracker.
 	if options.Verbose {
-		plugins.NewLogs(logger, interpreter)
+		plugins.NewLogs(logger, runtime)
 	}
 
 	// Run reduction.
-	interpreter.Run()
+	runtime.Run()
 
 	// Return the final reduced result.
-	result := interpreter.Expression().String()
+	result := runtime.Expression().String()
 	err = options.Destination.Write(result)
 	cli.HandleError(err)
 }

cmd/lambda/lambda_test.go

@@ -7,12 +7,12 @@ import (
 	"testing"
 
 	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
+	"git.maximhutz.com/max/lambda/pkg/normalorder"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 	"github.com/stretchr/testify/assert"
 )
 
-// Helper function to run a single sample through the lambda interpreter.
+// Helper function to run a single sample through the lambda runtime.
 func runSample(samplePath string) (string, error) {
 	// Read the sample file.
 	input, err := os.ReadFile(samplePath)
@@ -30,7 +30,7 @@ func runSample(samplePath string) (string, error) {
 	compiled := convert.SaccharineToLambda(ast)
 
 	// Create and run the reducer.
-	reducer := lambda.NewInterpreter(compiled)
+	reducer := normalorder.NewRuntime(compiled)
 	reducer.Run()
 
 	return reducer.Expression().String() + "\n", nil