<!-- Thanks for contributing. Fill out what applies. Delete sections that don't. -->

## What this PR does

<!-- One-sentence summary. -->

## Kind of change

- [ ] New lesson
- [ ] Fix to an existing lesson
- [ ] Translation
- [ ] New output (prompt, skill, agent, MCP server)
- [ ] Docs / website / tooling

## Checklist

- [ ] Code runs without errors with the listed dependencies
- [ ] No comments in code files (docs explain, code is self-explanatory)
- [ ] Built from scratch first, then shown with a framework (for new lessons)
- [ ] Lesson folder matches `LESSON_TEMPLATE.md` structure
- [ ] ROADMAP.md row for the lesson is a markdown link (`[Name](phases/...)`), not bare text
- [ ] One lesson per commit (atomic per-lesson rule)
- [ ] Tested locally / code output matches what `docs/en.md` claims

## Phase / lesson

<!-- e.g. Phase 5 · 03-tokenizers -->

## Notes for reviewer

<!-- Anything surprising, any deviations from the template, open questions. -->