flow: README — API reference + deterministic-replay contract

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>

plans/flow-on-sx.md

@@ -193,6 +193,12 @@ remote-node + crash-recovery + handoff + introspection together.
   registry. Timeout is a cooperative step budget (no wall clock in pure SX). Test
   harness reuses one env with a per-test reset for speed.
 
+- **Phases 5-7 + docs.** Operational API (introspection, hygiene), combinator
+  library (tap/recover/map-flow/while/until), railway `attempt`, end-to-end
+  integration suite, and `lib/flow/README.md` (full API reference + replay-semantics
+  contract). **151/151 across 10 suites.** Conformance sx_server timeout raised to
+  540s for the 10-suite run under shared-machine CPU contention.
+
 ## Blockers
 
 (none)