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().
