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/engine/engine.go

@@ -1,5 +1,5 @@
-// Package "engine" provides an extensible interface for users to interfact with
-// λ-calculus.
+// Package "engine" provides an extensible interface for users to interact with
+// λ-calculus and other expression evaluation modes.
 package engine
 
 import (
@@ -8,7 +8,7 @@ import (
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 )
 
-// A process for reducing one λ-expression.
+// A process for reducing one expression.
 type Engine struct {
 	Config     *config.Config
 	Expression *lambda.Expression
@@ -25,7 +25,7 @@ func New(config *config.Config, expression *lambda.Expression) *Engine {
 }
 
 // Begin the reduction process.
-func (e Engine) Run() {
+func (e *Engine) Run() {
 	e.Emit(StartEvent)
 
 	lambda.ReduceAll(e.Expression, func() {