feat(process): add reusable child supervisor

[joeykchen]

Summary

Extracts the driver-independent child-process lifecycle layer from #1753.

• Run one prepared exec.Cmd without changing the caller working directory, environment, or process-wide signal subscriptions.
• Preserve normal exit codes and Unix signal identity as structured status.
• Cancel and terminate the complete child process group with bounded escalation.
• Keep Unix and Windows behavior behind one small, reusable API.

This PR has no Project Driver, runtime bundle, cache, XGo, Mod, or release dependency.

Verification

go test -race ./internal/processsupervisor
go vet ./internal/processsupervisor
go test ./...

Follow-up

After merge, #1753 will rebase onto dev and remove this package from its diff. Runtime archive verification and transactional cache extraction are being prepared separately.

[fennoai]

Reviewed the complete fixed diff for the reusable process supervisor, including Unix process-group shutdown, cancellation/status handling, Windows job-object lifecycle, module changes, and tests. I found no reportable correctness, reliability, security, or compatibility regression introduced by this PR.

Verification completed: go test ./internal/processsupervisor, go test -race ./internal/processsupervisor, go vet ./internal/processsupervisor, GOOS=windows GOARCH=amd64 go test -c ./internal/processsupervisor, and go test ./....