0.2.1

Maintenance release: a tighter contributor workflow and simpler examples, with
no public API changes.

Highlights

• Adds pre-commit hooks that run make lint and make test before each commit,
  installed with make hooks and run across all files with make pre-commit.
• Removes the checked-in trace_snippet.json example fixtures; the example
  tests now assert trace-shape guarantees directly, so docs and examples no
  longer depend on static snapshot files.

Notes

No public API changes, so existing 0.2.0 code needs no migration. Contributors
should run make hooks once to install the new pre-commit hooks.