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M.V. Hutz c2aa77cb92 refactor: remove visitor pattern (#37 )

## Description

The codebase previously used the visitor pattern for traversing lambda calculus expressions.
This was a hold-over from avoiding the Go-idiomatic way of handling types.
This PR removes the visitor pattern in favor of direct method implementations.

- Remove `Visitor` interface from `expression.go`.
- Remove `Accept` methods from `Abstraction`, `Application`, and `Variable`.
- Remove `Accept` from `Expression` interface.
- Delete `stringify.go` and move `String()` logic directly into each type.
- Add compile-time interface checks (`var _ Expression = (*Type)(nil)`).
- Update `expr.Expression` to embed `fmt.Stringer` instead of declaring `String() string`.

### Decisions

- Moved `String()` implementations directly into each expression type rather than using a separate recursive function, as each type's string representation is simple enough to be self-contained.

## Benefits

- Simpler, more idiomatic Go code using type methods instead of visitor pattern.
- Reduced indirection and fewer files to maintain.
- Compile-time interface satisfaction checks catch implementation errors early.

## Checklist

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

Closes #36

Reviewed-on: #37
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

2026-01-17 20:46:07 +00:00