## Why
Code mode wall time should measure the host operation itself, without including
client-side response delays or idle time between requests.
## What changed
- Measure each execute, wait, and terminate request in the code mode host.
- Carry the duration through the stdio and gRPC protocols and use it for
model-visible wall time.
- Emit a structured `codex.code_mode.host_timing` event correlated with the
conversation, turn, tool call, and cell.
## Testing
- Cover successful and failed execution timing, delayed response reads,
repeated waits, termination, and missing cells across stdio and gRPC.
- Verify timing survives protocol serialization and is reflected in app-server
model output and structured telemetry.
GitOrigin-RevId: d24af30c3fc5820521b4beba1f9970714dad6482
## Summary
- implement the standalone `codex-code-mode-host` stdio service
- route sessions, cells, delegate requests, responses, and cancellation
through a bounded host peer
- supervise request, writer, cell-forwarding, actor, and V8 failure
boundaries
- bound request/session tombstones and fail-stop the connection on
invalid protocol state
- add host-only duplex protocol tests and local Cargo/Bazel run recipes
## Why
This stage makes the host process independently runnable and reviewable
before exposing any remote client in Codex. Transport or runtime failure
closes the connection and relies on process replacement rather than
transactional recovery.
## Stack
This is **3 of 4** in the process-owned code-mode session stack.
- Depends on #30110
- The final client PR targets this branch
## Validation
- `just test -p codex-code-mode-host` — 7 host-only tests passed
- `just fix -p codex-code-mode-host`
- `just bazel-lock-update`
- `just bazel-lock-check`
- `just fmt`
Adam Perry @ OpenAI