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().

pkg/expr/expr.go

@@ -0,0 +1,11 @@
+// Package expr provides the abstract Expression interface for all evaluatable
+// expression types in the lambda interpreter.
+package expr
+
+// Expression is the base interface for all evaluatable expression types.
+// Different evaluation modes (lambda calculus, SKI combinators, typed lambda
+// calculus, etc.) implement this interface with their own concrete types.
+type Expression interface {
+	// String returns a human-readable representation of the expression.
+	String() string
+}

pkg/lambda/expression.go

@@ -1,6 +1,11 @@
 package lambda
 
+import "git.maximhutz.com/max/lambda/pkg/expr"
+
+// Expression is the interface for all lambda calculus expression types.
+// It embeds the general expr.Expression interface for cross-mode compatibility.
 type Expression interface {
+	expr.Expression
 	Accept(Visitor)
 }
 
@@ -23,6 +28,10 @@ func (a *Abstraction) Accept(v Visitor) {
 	v.VisitAbstraction(a)
 }
 
+func (a *Abstraction) String() string {
+	return Stringify(a)
+}
+
 func NewAbstraction(parameter string, body Expression) *Abstraction {
 	return &Abstraction{parameter: parameter, body: body}
 }
@@ -46,6 +55,10 @@ func (a *Application) Accept(v Visitor) {
 	v.VisitApplication(a)
 }
 
+func (a *Application) String() string {
+	return Stringify(a)
+}
+
 func NewApplication(abstraction Expression, argument Expression) *Application {
 	return &Application{abstraction: abstraction, argument: argument}
 }
@@ -64,6 +77,10 @@ func (v *Variable) Accept(visitor Visitor) {
 	visitor.VisitVariable(v)
 }
 
+func (v *Variable) String() string {
+	return Stringify(v)
+}
+
 func NewVariable(name string) *Variable {
 	return &Variable{value: name}
 }