## Why
A process host should be discarded and rebuilt after critical actor or
V8 failure, while the existing in-process production path must keep its
current cell-error semantics. This change establishes that failure
boundary without adding the host process or remote client.
## What changed
- add optional task-failure supervision to the transport-neutral
code-mode session runtime
- report Tokio cell-actor failures and V8 runtime-thread panics to a
host-provided fail-stop handler
- preserve the existing handler-less in-process behavior
- make host-owned cell ID allocation fail before numeric wraparound
## Follow-up
The V8 panic signal surfaced here should also be consumed by the
`InProcessCodeModeSession` manager in a future change so it can fail the
affected cell. This PR intentionally leaves the handler-less in-process
behavior unchanged while putting the required panic tracking in place.
## Stack
This is **2 of 4** in the process-owned code-mode session stack.
- #30108 is merged into `main`
- The next PR targets this branch
## Validation
- `just test -p codex-code-mode` — 53 passed
- `just argument-comment-lint -p codex-code-mode`
- `just fix -p codex-code-mode`
Channing Conger
6c21297bba