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