refactor: Move default cluster startup to background

[rekmarks]

Moves the logic for conditionally starting the default subcluster from the kernel worker to the extension background. This is accomplished by means of making the ui-connections module agnostic of where it is used and refactoring it into internal-connections, establishing such a connection from the extension background, and using kernel RPC calls to conditionally launch the default subcluster. UI <-> kernel connections should work exactly the same as before.

The default subcluster is really just for testing, and we shouldn't enshrine it in kernel-browser-runtime, which we eventually want to use in production environments.

---

Note

Moves default subcluster launch from the kernel worker to the extension background via a new internal comms API, replacing ui-connections and updating consumers.

• Extension (packages/extension/src/background.ts):
  • Add startDefaultSubcluster() using connectToKernel and RPC (getStatus/launchSubcluster) to conditionally start the default subcluster.
  • Pipe offscreen responses via a single drainPromise and await it; minor init sequencing tweaks.
• Kernel Browser Runtime:
  • Internal Comms: Introduce internal-comms with connectToKernel, receiveInternalConnections, and comms-control-message (replaces ui-connections/ui-control-command).
  • Exports: Update src/index.ts and index.test.ts to expose new APIs.
  • Kernel Worker: Switch to receiveInternalConnections; remove first-run/default subcluster launch; keep remote comms init only.
  • RPC: Update launch-subcluster result type to nullable CapData.
  • Tests: Add/replace tests for internal comms and control message validation.
• Kernel UI (packages/kernel-ui/src/services/stream.ts):
  • Replace establishKernelConnection with connectToKernel (adds label).
• Tooling:
  • Adjust coverage thresholds in vitest.config.ts.

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[sirtimid]

LGTM!