update(chord-basic): altair — comprehensive review (#5215)

## Summary

Updated **altair** implementation for **chord-basic**.

**Changes:** Comprehensive review — code quality, data choice, visual
design, spec compliance, library feature usage.

## Test Plan

- [x] Preview images uploaded to GCS staging
- [x] Implementation file passes ruff format/check
- [x] Metadata YAML updated with current versions
- [ ] Automated review triggered

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Generated with [Claude Code](https://claude.com/claude-code) `/update`
command

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Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: MarkusNeusinger