Fix FOUC when navigating between hydrated client entries

[brophdawg11]

Summary

• Adopt server styles by refcount instead of resetting the adopted stylesheet on every reload.
• DOM preserved across a reload (e.g. the previous client entry's content inside a hydration boundary that diffNodes skips over) keeps its rules until the new module finishes loading and the virtual root re-renders, eliminating the flash of unstyled content.
• replaceServerStyles(source) on the style manager handles the prior/new diff, releasing only adoption refs of selectors absent from the next page — selectors held by an active css mixin (e.g. transient client-only state) are untouched.

Root cause

frame.ts used to call styleManager.reset() then adoptServerStyles() before diffNodes(). The synchronous block is fine for ordinary DOM, but diffNodes deliberately skips over hydrated client-entry boundaries and lets the hydration pass re-render them — that is an async step (module load). Between the style swap and the hydration re-render, the lingering previous-entry DOM had class names referring to rules reset() had just dropped.

Test plan

• pnpm --filter @remix-run/ui run test — 756 pass / 0 fail, including the new regression test in src/test/frame.test.tsx that gates module loading to assert the prior entry's rule stays live while its DOM is still visible
• pnpm --filter @remix-run/ui run typecheck
• oxlint clean on changed files
• Prettier clean on changed files
• Manual: in the docs site, navigate /api/remix/ui/button/demos/basic/ ↔ /api/remix/ui/button/demos/states/ and confirm the preview frame swaps without a visible flash

🤖 Generated with Claude Code

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The preview branch preview/pr-11402 has been deleted now that this PR is merged/closed.