refactor: extract abstract Expression interface

Introduce pkg/expr.Expression as a base interface for all evaluatable
expression types, enabling future support for multiple evaluation modes
(SKI combinators, typed lambda calculus, etc.).

- Add pkg/expr/expr.go with Expression interface requiring String() method.
- Update lambda.Expression to embed expr.Expression.
- Add String() method to Abstraction, Application, and Variable types.
- Update plugins to use String() instead of lambda.Stringify().

internal/plugins/explanation.go

@@ -1,4 +1,4 @@
-// Package "explanation" provides a observer to gather the reasoning during the
+// Package "explanation" provides an observer to gather the reasoning during the
 // reduction, and present a thorough explanation to the user for each step.
 package plugins
 
@@ -6,10 +6,9 @@ import (
 	"fmt"
 
 	"git.maximhutz.com/max/lambda/internal/engine"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
 )
 
-// Track the reductions made by a reduction proess.
+// Track the reductions made by a reduction process.
 type Explanation struct {
 	process *engine.Engine
 }
@@ -24,9 +23,9 @@ func NewExplanation(process *engine.Engine) *Explanation {
 }
 
 func (t *Explanation) Start() {
-	fmt.Println(lambda.Stringify(*t.process.Expression))
+	fmt.Println((*t.process.Expression).String())
 }
 
 func (t *Explanation) Step() {
-	fmt.Println("    =", lambda.Stringify(*t.process.Expression))
+	fmt.Println("    =", (*t.process.Expression).String())
 }