Test fixtures

These small Rust child programs are shared by the POSIX adapter and runtime acceptance tests. They invoke no shell or host utility and contain no unsafe code.

• process_observer.rs writes a length-prefixed binary report containing cwd, selected native environment values, descriptor visibility, and exact native argv. FLASH_PROBE_REPORT selects the report path, FLASH_PROBE_VALUE supplies the inspected environment value, and FLASH_PROBE_FD selects a descriptor.
• status.rs accepts exit CODE for ordinary completion or signal for a real SIGABRT termination.
• stream.rs provides deterministic source, relay, sink, both, and both-closed modes for stream, status, merged-output, and descriptor tests.
• benchmark.rs provides host-only in-process completion and lazy structured-stream probes for the versioned performance suite. It is not installed in the FlashOS image.