CSS Modules: `@extend` (postcss-extend-rule) silently dropped — CSS-Modules plugin runs before user PostCSS plugins

## Problem

`@extend` from `postcss-extend-rule` is silently discarded in CSS Modules. Every declaration the rule should have inherited is missing — in dev and in the production build alike — with no warning or error.

```css
/* styles.module.css */
.shared { position: absolute; visibility: hidden; }
.wrap { @extend .shared; border: 1px solid red; }
```

Expected: `.wrap` inherits `position: absolute; visibility: hidden`.

Actual output:

```css
._shared_1u9uh_1{visibility:hidden;position:absolute}
._wrap_1u9uh_2{border:1px solid red}
```

The at-rule is gone and nothing was copied.

## Cause

Vite's `compileCSS` puts the CSS-Modules plugin at the **front** of the PostCSS chain:

```js
if (isModule) postcssPlugins.unshift((await importPostcssModules()).default({ ... }))
```

So class names are already hashed to `._shared_1u9uh_1` by the time the project's `postcss-extend-rule` runs. `@extend .shared` matches no rule, and the plugin drops it silently.

Running the *same* PostCSS config standalone over the same file resolves the extend correctly, so the configuration is fine — only the ordering inside Vite's pipeline is wrong.

## Reproduction (no vinext involved)

```
postcss.config.mjs -> export default { plugins: { 'postcss-extend-rule': {} } }
src/styles.module.css -> the two rules above
src/main.js -> import s from "./styles.module.css"
vite build
```

The emitted CSS shows `_wrap_` without the extended declarations. This reproduces on plain Vite with no vinext in the project, so the underlying issue is Vite's plugin ordering — filing here because vinext already resolves and injects the project's PostCSS config, and is where a Next-compatible pipeline would be expected to handle it (webpack/`css-loader` runs the user's PostCSS before CSS-Modules scoping, so the same stylesheet works under `next build`).

## Impact

Any CSS module using `@extend` loses those declarations. In one Pages Router app this silently affected **46 `@extend` uses across 11 module files**, producing components with missing positioning — dropdown panels rendering in flow instead of as absolutely-positioned overlays, etc. Because nothing warns, it presents as "the CSS is just wrong" with no obvious cause.

## Workaround

Expanding `@extend` before Vite's pipeline sees the file, via an `enforce: "pre"` transform, fixes it — the extend resolves against the original selectors and Vite then hashes the already-expanded rules:

```ts
const expandCssModuleExtends = () => {
let processor;
return {
name: "expand-css-module-extends",
enforce: "pre" as const,
async transform(code: string, id: string) {
const file = id.split("?")[0];
if (!file.endsWith(".module.css") || !code.includes("@extend")) return null;
if (!processor) {
const postcss = (await import("postcss")).default;
const extendRule = (await import("postcss-extend-rule")).default;
const presetEnv = (await import("postcss-preset-env")).default;
// `@extend` must see flat selectors: postcss-extend-rule skips nested
// rules whose selector it cannot resolve, so flatten nesting first.
processor = postcss([
presetEnv({ stage: false, features: { "nesting-rules": true } }),
extendRule(),
]);
}
const result = await processor.process(code, { from: file });
return { code: result.css, map: null };
},
};
};
```

Note the nesting step — without it only the `@extend`s at one nesting depth are expanded, which looks like a partial fix.

## Environment

- vinext `1.0.0-beta.6`
- Vite `8.1.0`
- `postcss-extend-rule`, `postcss-preset-env`
- Reproduced in dev and in the production build, and on plain Vite without vinext

Start at Vite's compileCSS implementation and reproduce the issue with the minimal postcss.config.mjs, styles.module.css, and src/main.js setup described here. Trace how CSS Modules and user PostCSS plugins are ordered, then verify that @extend declarations survive in both dev and production builds, including nested rules.

Written by the indexing model from the issue text.