Carries a structured `ProcessError` when an error must surface through an exception. Raised by `Result.GetValueOrThrow()`, streaming infrastructure (JSON-line parsing, pump faults, and bounded streams in `StreamFullMode.Error` mode), and ProcessKit group initialization through DI. Streaming APIs may surface pipeline faults this way even when their run-result counterparts return `Result<_, ProcessError>`. `Message` is exactly `Error.Message`, so it inherits that render's guarantees: one line, with every caller-, child-, or peer-controlled fragment sanitized and bounded — safe to print into a log or a terminal even when a hostile child chose its own stderr. The full, unmodified payload is on `Error` (`Detail`, `Stdout`, `Stderr`, `Data`, `Original`), never truncated.