docs: make the UI component approach an explicit Day-1 decision

[DeyangChan]

What

The contracts assumed a headless UI foundation and all five stack packs pinned Radix, so the choice between headless primitives, a copy-in component set, and a styled kit was never put to the user. This makes it an explicit Day-1 decision and follows the consequences through the guide and the packs.

• apps/frontend/CLAUDE.md — new UI component approach section: ask before the first screen or component is built, record the answer under Learnings, and decide on how far the project's design guide sits from a kit's defaults. Four lines that presupposed a headless library (Component structure, Accessibility baseline, Interaction feedback, Coding standards) now defer to it.
• Design guide hydration — the guide is filled in per project rather than shipping foundations-only. Its Components & reuse chapter said components were "deliberately absent"; it now carries the hydration rules and an empty Specimens section. New Hydrating the design guide subsection defines the two moves and requires opening the rendered page for the user after either one.
• Stack packs — vercel-csr, vercel-ssr, mern, django, enterprise name Radix as their default instead of pinning it. vercel-ssr and enterprise additionally reject a styled kit built on runtime CSS-in-JS, which would pull the token boundary into a Client Component.
• Day-1 step 6 — sequences the decision, the token rebrand, the guide reconciliation, and the browser review. Folded into the existing step rather than added as a new one, to avoid renumbering.

Notes

No new abstraction, dependency, or config key. Docs and one HTML file only.

Precedence: the packs previously bound Radix as a plain binding line, which would have overridden the new decision for an adopting project. They now express it as a default, so the base decision governs and a styled kit is recorded as a deviation.

Test plan

• Template-integrity checks run locally against the working tree — whitespace invariants, relative-link resolution, pack required-files/precedence/conflict-register contract, add-on READMEs: 0 problems.
• design-guide.html parsed for tag balance after editing: no unbalanced or mismatched tags.
• Guide opened in a browser and the changed Components & reuse chapter reviewed as a rendered page.

Known issue, not fixed here

Root CLAUDE.md:158 refers to "Day-1 step 11", but the checklist has 9 steps and Protect main is step 9. Pre-existing; this change does not shift step numbering.

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