[Sannrox]

Closes #120

On timeout or a job-source abort, the runner writes a scratch cancel file
and waits a grace window before killing the process group. Engines that
expose cooperative cancel (mizuchi CancelToken) stop, return limit: cancelled, and the run is a typed Failure::Cancelled with that report.
The kill path still fires when the engine ignores the file; the child is
gone within five seconds. Engines without a token stay on the kill path.

Cancelled local runs write failure.txt and a receipt whose
engine_version is the engine report, with empty outputs.

Reuse decision

Reuse the existing sandbox wait/kill, mizuchi CancelToken at the worker
edge, and Failure / receipt reporting. The engine-agnostic signal is a
scratch cancel file (not SIGTERM: that killed linux_native mizuchi
before it could report). JobSource still has no execute-time cancel
hook; the sandbox accepts an abort flag for when one exists. Kill after
grace stays Failure::Limit.

Lane

• claim branch: feat/120
• worktree: .worktrees/issue-120
• base SHA: 5f8aa37e8f95d77adbd603558bdeb32b45103d91
• published SHA: 482eeb7c46ef6c4d6abf67edd30720b29009d854
• authority: land

Verification

• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo clippy --locked -p kako-source-sekai --features sekai-client --all-targets -- -D warnings
• cargo test --locked --workspace
• cargo test --locked -p kako-source-sekai --features sekai-client
• bash scripts/validate-workflow-skills.sh
• python3 scripts/adr-index --check

Focused: cooperative cancel returns the engine report without kill;
ignored cancel kills within five seconds; source abort uses the same
path; mizuchi timeout is typed cancelled; cancelled receipt carries the
report; exclusive cancel-file create does not follow a symlink.

Skipped: kekkai-guest userspace payload tests (helper not on PATH).
validate-behavior: skipped; the MVP and sandbox tests observe the
receipt, failure.txt, and process lifetime.

Autoreview

AUTOREVIEW_CODEX_CONFIG=model_provider=cliproxyapi "$AUTOREVIEW" --mode local --engine codex

Clean: no accepted/actionable findings.

Simplify-change

Userspace spawn uses the same PID-aware wait/kill path as in-process
backends (no watcher around kekkai's blocking deadline helper). Capture
stays a 4 MiB head drain per stream. Receipt identity reuses
engine_version rather than a schema bump.

Compatibility

• JobSource, receipt schema, and the kekkai pin are unchanged.
• Default CPU when unset is 2 * wall_clock + cancel grace so RLIMIT_CPU
  does not kill a cooperative engine first.
• Successful jobs that print limit: cancelled still complete; only a
  failed worker exit with that report is typed cancelled.