docs(react): give the README's cascade layer an order that works (#102)

## Summary

`packages/react/README.md` told hosts to write this, under a sentence
about letting the package win:

```css
@layer domainkit, app;
```

A layer's priority is its position in that list, so `app` wins. The
example did the opposite of what
the prose claimed, and a host following it to keep the package's styles
got the reverse.

## Now

```css
@layer theme, base, components, domainkit, utilities;
```

`domainkit` sits after `base` so Tailwind's preflight cannot strip the
package's buttons back to
bare text, and before `utilities` so a utility class still wins. Beating
a part is a layer declared
*after* `domainkit`:

```css
@layer utilities {
  [data-domainkit-part="records-panel"] {
    border-radius: 0;
  }
}
```

Wording and order match `examples/react/styles.css` (`layer` and
`override` regions) and the
reference page from #101, so the README and the docs site say the same
thing.

## Scope

README prose only. No source, no stylesheet, no changelog: the README
ships with the 0.9.1 tarball.

`bun run --filter @domainkit/react release:check` is green (41 unit, 5
artifact, 10 Playwright),
with lint and format.