refactor: rename interpreter to runtime and use receiver methods

- Rename pkg/interpreter to pkg/runtime
- Move ReduceOnce to new pkg/normalorder package
- Convert standalone functions (Substitute, Rename, GetFree, IsFree)
  to receiver methods on concrete expression types
- Change Set from pointer receivers to value receivers
- Update all references from interpreter to runtime terminology

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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
 }