@tailwindcss/postcss: type error when trying to use with latest postcss

**What version of Tailwind CSS are you using?**

v4.3.1

**What build tool (or framework if it abstracts the build tool) are you using?**

postcss@8.5.16

**What version of Node.js are you using?**

v24.18.0

**What browser are you using?**

Irrelevant

**What operating system are you using?**

Fedora 44, but issue occurs anywhere

**Reproduction Steps**

```
mkdir tailwindcss-postcss-type-issue
cd tailwindcss-postcss-type-issue
npm init -y
npm pkg set type=module
npm i postcss typescript -D
npx tsc --init
npm i @tailwindcss/postcss -D
echo "import tailwindPostcss from '@tailwindcss/postcss'; import postcss from 'postcss'; postcss([tailwindPostcss()]);" > main.ts
npx tsc --noEmit
```

**Describe your issue**

Trying to use `@tailwindcss/postcss` with latest `postcss@8.5.16` version results in the following type error:
```
error TS2769: No overload matches this call.
  Overload 1 of 2, '(plugins?: readonly AcceptedPlugin[] | undefined): Processor_', gave the following error.
    Type 'Processor_ | Plugin' is not assignable to type 'AcceptedPlugin'.
    ...
```

This happens because `@tailwindcss/postcss'` pins postcss to v8.5.15, whose types are not forward compatible with v8.5.16.

This can be worked around when using npm by forcing newer postcss to get installed:
```jsonc
// package.json
{
// ...
  "overrides": {
    "@tailwindcss/postcss": {
      "postcss": "^8.5.16"
    }
  }
}
```

Possible solutions:
- bump pinned postcss dependency to 8.5.16 (may require future bumps for future versions)
- use a caret for version range, as postcss respects semver

IMO caret is preferred, but this is at your discretion.

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