[beinan]

Summary

A timed-out remote command can leave an ingestion experiment running. Add an optional Linux Rust runner that claims each operation directory once, detaches the child from terminal hangups, and records process identity, completion and log hashes. An unknown outcome stays observable and cannot authorize another launch; explicit stop uses a pidfd and incarnation checks.

This is an isolated experiment tool. It does not change production ingestion or supervise a service automatically. CI explicitly exercises the optional feature.

Refs #378.

Testing

• Four real-process regressions passed: racing launches, supervisor loss/orphan observation, checksum/claimed-directory refusal, and hangup/explicit-stop behavior. The ignored child fixture is invoked by these tests.
• Optional-feature release Clippy, formatting, spelling and whitespace checks passed.
• Full repository checks passed: workspace formatting, Clippy and all-target Rust tests (594 passed); Python tests (229 passed, 2 skipped, 1 xfailed), Ruff formatting/lint and Pyright.