Theming P2: make the Tailwind theme token-aware #29

Design: docs/design/2026-08-24-theming-tokens-and-slots.md

Follows the Bootstrap bridge. Tailwind compiles colour into utility classes, so bg-indigo-600 cannot be re-pointed by a CSS variable the way Bootstrap's component variables can.

Making the Tailwind theme token-aware means rewriting its class map to arbitrary values:

button: { variants: { primary: 'bg-[var(--rnx-primary)] text-white' } }

This needs no Tailwind config change and no build step for CDN users, but it touches every entry in themes/tailwind/index.js.

Do not start this while #4, #8, #9 and #11 are open. Those are existing Tailwind theme defects. Rewriting the class map on top of them puts two unrelated causes in play at once, and any failure becomes ambiguous.

Depends on the token names being final, so it waits for the P1 issue to land.