v1.2.0 — WCAG contrast audit & background tints

Added

• Background tints (bg-1…bg-5) per theme: 25 hue-distinct ultra-light colors for region backdrops, swimlane zebra striping, and filled group boxes — closes the gap where large diagrams had to invent off-theme pastels; the validator recognizes them
• tree coordinate walkthrough in examples.md (bottom-up: children first, parent centered over children's span)

Changed

• Contrast audit: all 5 themes re-tuned against WCAG targets — text roles ≥ 4.5:1, arrow lines ≥ 3:1, node strokes ≥ 2.5:1 (group borders exempt as decorative). 36 values darkened one step in the same hue; theme identities preserved. Biggest fixes: arrow colors (previously 2.0–2.6:1 across all themes) and morandi's stroke family
• Bundled example .drawio files recolored attribute-aware; all six re-validate clean
• tree layout algorithm clarified in references/layouts.md
• Swimlane zebra striping now uses bg-* tints