User-facing docs for the flow engine: the node model, every combinator, the
suspend/resume durability contract (escape-only call/cc -> deterministic replay),
lifecycle/introspection/hygiene API, fed-sx distribution, and substrate notes.
Doc-only; 151/151 unchanged.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
