Build a `ProcessResult` with full control over stderr, the `Outcome` (e.g. `Outcome.Signalled` / `Outcome.TimedOut`), and the duration. Success is judged against the default `{0}` ok-codes.

Build a failed `ProcessResult` (a non-zero exit) for tests. `exitCode` must be non-zero — `0` is rejected because the result is judged against the default `{0}` ok-codes, so a zero exit would make `IsSuccess` `true` (use `Success` for that). Negative codes are allowed (Windows reports them).

Build a successful `ProcessResult` (exit 0, given stdout) for tests.

Demand a successful run (an **accepted** exit code — one in `Command.OkCodes`, `{0}` by default): returns the result unchanged on success, otherwise the corresponding `ProcessError` (`Exit` / `Signalled` / `Timeout`). Generic over the captured-stdout type.