[t-kalinowski]

Adds serve --jupyter-kernel NAME so Console can use an installed local Jupyter kernel while retaining its MCP interface and sandbox. This makes published ark and ipykernel available as optional execution backends; the built-in worker remains the default.

• Exposes native source through send(code=...), including the synchronous and asynchronous Python clients. Kernel sessions support persistent state, text and image output, prompted input, interrupts, restart, and source archives.
• Runs ark and ipykernel under the default read-only sandbox without manual permission changes. macOS uses Unix sockets beneath the runner's private temporary directory; Linux uses loopback inside an isolated network namespace with a deny-all proxy. Host networking remains blocked.
• Embeds a Python adapter that owns and reaps the kernel child, correlates Jupyter replies with idle messages, preserves channel reads across startup, and keeps signed socket traffic asynchronous. The sandbox companion pin is unchanged, and no kernel source is patched or bundled.
• Keeps exact transcript completion markers for combined control and cell calls, including Jupyter code, across split responses. A gated native startup case exercises the split while preserving its sandbox and package-preparation assertions. These collector changes affect tests only.
• Documents setup, lifecycle and output limitations, history/export behavior, performance measurements, and external installation and binary-size costs. Website checks require the new guide in the task table and Guides sidebar.

This mode requires preinstalled packages and supports macOS/Linux. It does not provide Console's managed dependency preparation, SQL sessions, Windows worker support, or interactive notebook displays. Reticulate integration is in the dependent draft #179.

Local feature validation on macOS passed all 13 applicable ipykernel and ark cases, 22 lifecycle/CLI executions, all 36 website checks, scripts/format, and scripts/check. Seven affected baseline SQL/startup cases also passed in 12 executions. Two Linux kernel cases were capability-skipped locally. Benchmark data retains its original measured revision and is not presented as a rerun of this branch.

The hosted Unix runners capability-skip the optional Jupyter execution cases when kernel dependencies are not configured; those runs do not establish kernel execution coverage. The Windows runner reports MXC unavailable, so native sandbox execution remains unverified. Documentation deployment is intentionally skipped for pull requests.

For the collector repair, a public ipykernel restart with bundled code="pass" and timeout_ms=0 reproduced the missing completion marker and passed after the fix, followed by an accepted cell. The native R startup gate, ipykernel interrupt/restart case, and ark output/error case passed on macOS (three cases). scripts/format, git diff --check, and scripts/check passed. Current-head hosted checks remain pending; Linux and Windows were not rerun locally for this repair.