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>

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())
 }