Fix shared Turbopack runtime initialization race (#97215)
## Summary
- Allow the shared browser runtime to initialize before any async chunk
has created the chunk-loading queue.
- Preserve duplicate-runtime protection when a runtime registry is
already installed.
- Update the affected development, production, debug-ID, and worker
runtime snapshots.
Shared runtime scripts can execute before other async chunks.
Previously, the runtime returned when the chunk-loading global was
undefined, leaving later registrations queued and preventing the Next.js
client bootstrap and hydration from running.
## Verification
- `cargo test -p turbopack-ecmascript-runtime`
- Targeted Turbopack browser-runtime snapshot fixtures
- Generated development and production runtimes evaluated with no
pre-existing queue, including duplicate-runtime protection
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