App Router metadata icons: favicon/icon links not emitted and reinjection script missing

> *This issue was created by an agent analysing CI failures from the [Next.js Deploy Suite](https://github.com/cloudflare/vinext/actions/runs/26282973805) (vinext `main` vs Next.js `v16.2.6`, 2026-05-22).*

## Problem

App Router metadata icon support is incomplete. Automatic favicon detection emits zero `` elements when one is expected; custom icons declared via metadata (`shortcut-icon.png`) do not appear in the document head; the runtime script that moves body-injected icon links back into the head during streaming is missing.

```
Expected: 1 (favicon link count) | Received: 0
```

```
Expected: "/shortcut-icon.png" | Received: undefined
```

## Estimated Impact

~6 test failures across the deploy suite.

## Affected Test Suites

- `test/e2e/app-dir/metadata-icons/metadata-icons.test.ts` (6 failures)

## Recommendation

1. **Reproduce first in vinext's own test suite.** Add a fixture with `app/icon.png` and `app/shortcut-icon.png` and assert both render as `` / `` in the head.

2. **Implement automatic icon detection.** Scan the `app/` directory for the documented icon filenames and emit the corresponding head tags.

3. **Implement metadata-driven icons.** Honor `metadata.icons` in `layout.tsx` / `page.tsx`.

4. **Add the body->head reinjection script.** During streaming, icon links may flush into the body; emit the small reinjection script Next.js ships so the browser moves them to the head.