Switch kernel launch to IPC transport (supersedes #1772)

[rgbkrk]

Replace the current port-reservation-based kernel launch (which has a TOCTOU race around port reuse — the original #1772 issue) with IPC transport (Unix domain sockets / named pipes) for the kernel's Jupyter control/shell/iopub/stdin/heartbeat channels.

Why now

• The TCP port-retry dance is fragile: we pick a port, kernel may bind late, port collisions cause silent failures or require restart.
• IPC sockets have no port namespace collision surface.
• jupyter_client supports IPC transport (transport: "ipc") — kernel-side wiring is minimal.
• Aligns with runtimed-agent architecture (already Unix-socket based).

Work

1. Design spec: what exactly do we change in kernel-launch, JupyterKernel, and the connection-info JSON written to disk? Where do the 5 IPC socket files live (daemon base dir? worktree-scoped?)?
2. Implementation in crates/kernel-launch + crates/runtimed/src/jupyter_kernel.rs.
3. Per-kernel cleanup: socket files need to be removed on kernel shutdown AND orphan-reaped by process_groups if the kernel crashes.
4. Windows story: jupyter_client's IPC support uses named pipes — verify it works or keep TCP for Windows.

Scope gate

Prerequisite: confirm ipykernel honors transport: ipc on all supported Python versions. If there's any gap, falls back to TCP there.

Related

• fix(runtimed): add wall-clock launch deadline and retry for kernel port collisions #1772 (closed in favor of this)