M.V. Hutz da3da70855 refactor: extract shared token package (#46) ...

## Description

Both the `saccharine` and `lambda` packages need tokenizing and parsing primitives.
This PR extracts shared token infrastructure into a new `pkg/token` package, then wires both languages up to use it.

- Add `pkg/token` with a generic `Token[T]` type, `Scan`, `ScanAtom`, `ScanRune`, `ScanCharacter`, `IsVariable`, `ParseRawToken`, and `ParseList`.
- Refactor `pkg/saccharine` to delegate to `pkg/token`, removing duplicated scanning and parsing helpers.
- Implement `Codec.Decode` for `pkg/lambda` (scanner + parser) using the shared token package.
- Add `iterator.While` for predicate-driven iteration.
- Rename `iterator.Do` to `iterator.Try` to better describe its rollback semantics.

### Decisions

- The `Type` constraint (`comparable` + `Name() string`) keeps the generic token flexible while ensuring every token type can produce readable error messages.
- `iterator.Do` was renamed to `iterator.Try` since it describes a try/rollback operation, not a side-effecting "do".

## Benefits

- Eliminates duplicated token, scanning, and parsing code between languages.
- Enables the `lambda` package to decode (parse) lambda calculus strings, which was previously unimplemented.
- Makes it straightforward to add new languages by reusing `pkg/token` primitives.

## Checklist

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

Reviewed-on: #46
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

2026-02-12 00:25:18 +00:00

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