unhead: Streaming SSR `streamKey` injected into inline script without identifier validation

Summary

### Summary

`createStreamableHead({ streamKey })` interpolated its `streamKey` argument directly into the streaming SSR bootstrap and suspense-chunk inline scripts without identifier validation or escaping. If an application forwards untrusted data into that configuration value, the rendered scripts become a script-injection sink.

### Details

`streamKey` was embedded into JavaScript source via dot notation in two public helpers:

* `createBootstrapScript()` returned `<script>window.${streamKey}={...}</script>` * `renderSSRHeadSuspenseChunk()` returned `window.${streamKey}.push(...)`

No escaping, quoting, or identifier validation was applied before these strings were embedded into HTML. A `streamKey` such as `__unhead__;globalThis.PWNED=1;//` broke out of the intended property access and injected arbitrary JavaScript into the page. The JSON escaping used for streamed head entries did not protect `streamKey` because `streamKey` was inserted as raw code rather than as serialized data.

### Impact

`streamKey` is a developer-chosen configuration value rather than a data field — the intended usage is a hardcoded identifier-shaped constant (default `__unhead__`). Exploitation therefore requires an application to explicitly route untrusted input into a configuration sink, which is not a documented or recommended pattern. We have no reports of any downstream project sourcing `streamKey` from request data.

Applications using the default `streamKey`, or any hardcoded custom key, are **not affected**.

### PoC

```ts import { createStreamableHead, renderSSRHeadShell } from 'unhead/stream/server'

const { head } = createStreamableHead({ streamKey: '__unhead__;globalThis.PWNED=1;//', })

const html = renderSSRHeadShell( head, '<!doctype html><html><head></head><body></body></html>', )

// <!doctype html><html><head><script>window.__unhead__;globalThis.PWNED=1;//={_q:[],push(e){this._q.push(e)}}</script>… ```

### Patch

Fixed on `main` in [`64b5ac0`](https://github.com/unjs/unhead/commit/64b5ac0aa30cc256ea6677ce3dc4f132f81b2ff6). The fix will ship in the next patch release of `unhead`.

`streamKey` is now validated against a conservative ASCII JavaScript-identifier pattern (`/^[$_a-z][$\w]*$/i`) at every sink — `createStreamableHead`, `createBootstrapScript`, and the internal stream-key resolver. Invalid values throw immediately instead of being emitted into script output.

### Workarounds

Do not pass untrusted data into `createStreamableHead({ streamKey })` or `createBootstrapScript(key)`. If per-tenant keys are required, whitelist them against an identifier-safe pattern before constructing the head instance.

### Credit

Thanks to @Jvr2022 for the report.