refactor: extract abstract Expression interface (#30)

## Description

The codebase currently couples the engine and plugins directly to `lambda.Expression`.
This PR introduces an abstract `expr.Expression` interface to enable future support for multiple evaluation modes.

- Add `pkg/expr/expr.go` with an `Expression` interface requiring a `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()`.

### Decisions

- The `expr.Expression` interface is minimal (only `String()`) to avoid over-constraining future expression types.
- The engine still stores `*lambda.Expression` directly rather than `expr.Expression`, because Go's interface semantics require pointer indirection for in-place mutation during reduction.
- Future evaluation modes will implement their own concrete types satisfying `expr.Expression`.

## Benefits

- Establishes a foundation for supporting multiple evaluation modes (SKI combinators, typed lambda calculus, etc.).
- Plugins now use the abstract `String()` method, making them more decoupled from the lambda-specific implementation.
- Prepares the codebase for a Reducer interface abstraction in a future PR.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`).
- [x] Tests pass (if applicable).
- [ ] Documentation updated (if applicable).

Reviewed-on: #30
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

cmd/lambda/lambda.go

@@ -8,7 +8,6 @@ import (
 	"git.maximhutz.com/max/lambda/internal/engine"
 	"git.maximhutz.com/max/lambda/internal/plugins"
 	"git.maximhutz.com/max/lambda/pkg/convert"
-	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
 )
 
@@ -32,7 +31,7 @@ func main() {
 
 	// Compile expression to lambda calculus.
 	compiled := convert.SaccharineToLambda(ast)
-	logger.Info("compiled λ expression", "tree", lambda.Stringify(compiled))
+	logger.Info("compiled λ expression", "tree", compiled.String())
 
 	// Create reduction engine.
 	process := engine.New(options, &compiled)
@@ -62,7 +61,7 @@ func main() {
 	process.Run()
 
 	// Return the final reduced result.
-	result := lambda.Stringify(compiled)
+	result := (*process.Expression).String()
 	err = options.Destination.Write(result)
 	cli.HandleError(err)
 }