Offer a Design Book-audited shell theme family

[flyingrobots]

Idea

Offer a Bijou FrameShellThemeFamily with deliberately related dark and light modes so operators can switch appearance without losing the git-cas identity or accessibility guarantees.

Product contract

• Select every text/background pair with Design Book.
• Require at least WCAG AA 4.5:1 contrast for normal text; prefer 7:1 for primary and subdued operational copy where the palette permits it.
• Register every semantic pair with Bijou safe-pair metadata and keep doctorTheme() regression coverage.
• Preserve meaning without color through labels, symbols, hierarchy, and linear reading order.
• Use Bijou's shell theme settings and mode switch; do not add another app-owned theme overlay.
• Treat terminals without truecolor support and NO_COLOR as first-class lower modes.

Exploration questions

• Can one hue family preserve recognizable warning, success, info, and danger roles across both modes?
• Which surfaces should remain application-owned versus frame-owned?
• Can a screenshot/witness matrix cover common terminal capability profiles without becoming a brittle golden suite?

This is a non-blocking v6.6.0 exploration. The v6.5.6 frame conversion intentionally ships only the audited dark theme.