[Foundation] Migrate to new beta colour tokens

Part of #5119 (phase 1 — colour). Follows the migration pattern established in Button (#5208) and Chip (#5223).

## Scope

- Switch the foundation styles (`/next` `Foundation/` — `elements.css` etc.) onto the expanded static colours — step-based state names (#4742, #4744)
- Use semantic tokens where they exist and fit; flag new gaps as learnings

## Definition of Done

- [ ] Foundation uses the new beta colour tokens (no references to the old colour tokens)
- [ ] Visual regression checked against current Foundation (light/dark)
- [ ] Learnings captured on #5208/#5221 if new gaps surface

Start in the /next Foundation/ styles, including elements.css, and compare the migration patterns in Button (#5208) and Chip (#5223). Replace the old colour-token references with suitable beta tokens, then run the Foundation visual regression checks in light and dark modes and record any new gaps on #5208 or #5221.

Written by the indexing model from the issue text.