Summary

• add a Vite/React gallery that applies the unchanged @opencode-ai/css theme from feat(css): add framework-agnostic design system #38322 to Base UI primitives
• demonstrate fields, select, checkbox, switch, slider, tabs, accordion, tooltip, popover, menu, and dialog behavior
• emit a single-file production build so the gallery can run inside sandboxed artifact hosting

Checks

• bun run typecheck from packages/css-base-ui
• bun run build from packages/css-base-ui
• bun test from packages/css (5 passed)
• Chromium at 1440×1050/dark: select, tabs, accordion, popover, menu, and dialog interactions passed with no console or page errors
• Chromium at 390×844/dark: no horizontal overflow
• built artifact served and interaction-tested from the sandboxed artifact URL

Stack

• stacked on feat(css): add framework-agnostic design system #38322 so this PR contains only the Base UI experiment

Requested by: @jlongster (James Long via Slack)