Missing pvector.dist

[PeteHaughie]

Summary

PVector was missing dist method

Related issues

N/A

Checklist

• I added tests covering the new behavior (or gave a clear reason why not).
• I ran mypy src/ locally and fixed type issues.
• I ran ruff check src/ tests/ and addressed any findings.
• I ran the test suite locally: pytest -q.
• I updated docs/ where user-visible behavior changed.
• If this is a migration from a monolithic module into a package, a compatibility shim that re-exports the public API is included and noted here.

For contribution guidance, see CONTRIBUTING.md and docs/package-style.md.

Additional notes for reviewers:

• Design decisions:
• Potential follow-ups: