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>

internal/plugins/debug.go

@@ -3,17 +3,17 @@ package plugins
 import (
 	"log/slog"
 
-	"git.maximhutz.com/max/lambda/pkg/interpreter"
+	"git.maximhutz.com/max/lambda/pkg/runtime"
 )
 
 type Logs struct {
 	logger  *slog.Logger
-	reducer interpreter.Interpreter
+	reducer runtime.Runtime
 }
 
-func NewLogs(logger *slog.Logger, r interpreter.Interpreter) *Logs {
+func NewLogs(logger *slog.Logger, r runtime.Runtime) *Logs {
 	plugin := &Logs{logger, r}
-	r.On(interpreter.StepEvent, plugin.Step)
+	r.On(runtime.StepEvent, plugin.Step)
 
 	return plugin
 }

internal/plugins/explanation.go

@@ -5,19 +5,19 @@ package plugins
 import (
 	"fmt"
 
-	"git.maximhutz.com/max/lambda/pkg/interpreter"
+	"git.maximhutz.com/max/lambda/pkg/runtime"
 )
 
 // Track the reductions made by a reduction process.
 type Explanation struct {
-	reducer interpreter.Interpreter
+	reducer runtime.Runtime
 }
 
 // Attaches a new explanation tracker to a reducer.
-func NewExplanation(r interpreter.Interpreter) *Explanation {
+func NewExplanation(r runtime.Runtime) *Explanation {
 	plugin := &Explanation{reducer: r}
-	r.On(interpreter.StartEvent, plugin.Start)
-	r.On(interpreter.StepEvent, plugin.Step)
+	r.On(runtime.StartEvent, plugin.Start)
+	r.On(runtime.StepEvent, plugin.Step)
 
 	return plugin
 }

internal/plugins/performance.go

@@ -7,7 +7,7 @@ import (
 	"path/filepath"
 	"runtime/pprof"
 
-	"git.maximhutz.com/max/lambda/pkg/interpreter"
+	"git.maximhutz.com/max/lambda/pkg/runtime"
 )
 
 // Observes a reduction process, and publishes a CPU performance profile on
@@ -19,10 +19,10 @@ type Performance struct {
 }
 
 // Create a performance tracker that outputs a profile to "file".
-func NewPerformance(file string, process interpreter.Interpreter) *Performance {
+func NewPerformance(file string, process runtime.Runtime) *Performance {
 	plugin := &Performance{File: file}
-	process.On(interpreter.StartEvent, plugin.Start)
-	process.On(interpreter.StopEvent, plugin.Stop)
+	process.On(runtime.StartEvent, plugin.Start)
+	process.On(runtime.StopEvent, plugin.Stop)
 
 	return plugin
 }

internal/plugins/statistics.go

@@ -6,7 +6,7 @@ import (
 	"time"
 
 	"git.maximhutz.com/max/lambda/internal/statistics"
-	"git.maximhutz.com/max/lambda/pkg/interpreter"
+	"git.maximhutz.com/max/lambda/pkg/runtime"
 )
 
 // An observer, to track reduction performance.
@@ -16,11 +16,11 @@ type Statistics struct {
 }
 
 // Create a new reduction performance Statistics.
-func NewStatistics(r interpreter.Interpreter) *Statistics {
+func NewStatistics(r runtime.Runtime) *Statistics {
 	plugin := &Statistics{}
-	r.On(interpreter.StartEvent, plugin.Start)
-	r.On(interpreter.StepEvent, plugin.Step)
-	r.On(interpreter.StopEvent, plugin.Stop)
+	r.On(runtime.StartEvent, plugin.Start)
+	r.On(runtime.StepEvent, plugin.Step)
+	r.On(runtime.StopEvent, plugin.Stop)
 
 	return plugin
 }