Add 0.4 track interface docs and rendering fixes

[ginitheclaw]

Summary

• update the tutorial for the 0.4.0 track interface and switch repository tutorial figures to Matplotlib output
• fix per-track track_bg, track-specific color handling, and track-first legends across Matplotlib and Plotly
• reserve vertical label space for above/below labels and refresh visual/JSON baselines
• add example SNP data and grouped print_options() output

Tests

• .venv/bin/python -m pytest tests -q
• .venv/bin/sphinx-build -b html docs docs/_build/html

Version is already set to 0.4.0 in src/pyrangeyes/_version.py.