Read the CSS named colors that ReportLab lacks or defines differently - #542

Comparing ReportLab's named colors with the CSS list (148 names, from tinycss2), three differ:

• rebeccapurple (#663399, CSS Color 4) and lightgray (#d3d3d3; ReportLab only has lightgrey) are missing, so fill="rebeccapurple" leaves the shape without a fill and logs Can't handle color.
• darkseagreen is #8fbc8b in ReportLab but #8fbc8f in CSS.

convertColor() now looks these three up in a small _CSS_NAMED_COLORS table before falling back to ReportLab's colors. I didn't use tinycss2.color4 for this because svglib still allows tinycss2 0.6, which doesn't have it.

Added test_css_named_colors_missing_in_reportlab; it fails on main. Updated tests/public_api.json with --update for the new private name, and added a ### Fixed entry in CHANGELOG.md (I used #542, I'll fix it if the PR gets another number). pytest, ruff format --check, ruff check and mypy src tests pass.

AI-assisted (Claude Code); I reviewed and tested the change