Implement APCA algorithm for light theme readability scoring (litmus-u02e)

Replace WCAG 2.x symmetric contrast ratio with APCA (polarity-aware) in readability_score(). WCAG under-estimates perceived contrast for dark text on light backgrounds, causing light themes to score incorrectly low. APCA correctly models both polarities. Results: Catppuccin Latte 31.9% → ~90%, Solarized Light 10.2% → 99.6%, with parity to dark variants.