docs(react): give the cascade layer an order that survives preflight (#101)

## Summary

The styling page named the layer but not the order, and an undeclared
layer lands wherever it first
appears. `domainkit` can sort under Tailwind's preflight, and the
package's buttons come out as bare
text.

## The order

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

`domainkit` sits after `base` so the reset cannot strip it, and before
`utilities` so a utility
class still wins. It is on the React styling page and the
`@domainkit/react` reference, from
`examples/react/styles.css`.

## A correction alongside it

The old example claimed to show the package winning while ordering a
host layer above it:

```css
/* before — labelled "letting the package win", but `app` wins */
@layer domainkit, app;

/* after — the override, named for what it is */
@layer utilities { ... }
```

## Validation

- `bun run release:check` (root, React leg included)
- `bun run typecheck:examples`
- `bun run --filter @domainkit/docs typecheck`, `test`,
`reference:check`, `build`
- `blume validate --strict` (no broken links)
- `blume audit --strict` (3,724 audits, 0 errors, 0 warnings)