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/debug.go

@@ -4,7 +4,6 @@ import (
 	"log/slog"
 
 	"git.maximhutz.com/max/lambda/internal/engine"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
 )
 
 type Logs struct {
@@ -20,6 +19,5 @@ func NewLogs(logger *slog.Logger, process *engine.Engine) *Logs {
 }
 
 func (t *Logs) Step() {
-	stringified := lambda.Stringify(*t.process.Expression)
-	t.logger.Info("reduction", "tree", stringified)
+	t.logger.Info("reduction", "tree", (*t.process.Expression).String())
 }