### What?

Metadata routes (`manifest.ts`, `robots.ts`, `sitemap.ts`, `icon.tsx`,
`apple-icon.tsx`, etc.) were not being hot-reloaded in Turbopack dev
mode — changes to those files would not be reflected on subsequent
requests until a full server restart.

### Why?

PR #91466 extended `usesServerHmr = true` in `clearRequireCache()` (in
`hot-reloader-turbopack.ts`) from `app-page` entries only to **all**
`app`-type entries (pages + route handlers). The motivation was correct:
regular route handlers like `app/api/hello/route.ts` use Turbopack's
in-place module update model and benefit from server HMR.

However, metadata routes (`/manifest.webmanifest/route`,
`/robots.txt/route`, etc.) are also `app`-type entries but they are
**not** suitable for in-place server HMR. When `usesServerHmr = true`
for a metadata route, `clearRequireCache()` skips two critical
invalidation steps:

1. Deleting the compiled chunk from `require.cache`
2. Calling `__next__clear_chunk_cache__()`

Without those steps, the old module stays in-memory and all subsequent
requests to `/manifest.webmanifest` (etc.) return the stale content.

### How?

Added an `!isMetadataRoute(entryPage)` guard to the `usesServerHmr`
expression in `clearRequireCache()`. This restores full cache
invalidation for metadata routes on every rebuild while leaving regular
route handler server HMR (added in #91466) intact.

```ts
// Before
const usesServerHmr =
  serverFastRefresh &&
  entryType === 'app' &&
  writtenEndpoint.type !== 'edge'

// After
const usesServerHmr =
  serverFastRefresh &&
  entryType === 'app' &&
  writtenEndpoint.type !== 'edge' &&
  !isMetadataRoute(entryPage)   // ← metadata routes always clear the cache
```

`isMetadataRoute('/manifest.webmanifest/route')` → `true` (excluded from
server HMR)
`isMetadataRoute('/api/hello/route')` → `false` (keeps server HMR, no
regression)

Also added a regression test: `metadata route hmr > reflects manifest.ts
changes on fetch/refresh` in the `server-hmr` test suite, with a
`manifest.ts` fixture that starts at `name: 'Version 0'`. The test
patches the file and asserts the updated JSON is returned on the next
fetch.

Fixes #91981

---------

Co-authored-by: Will Binns-Smith <wbinnssmith@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>