Guest Scheme call/cc is escape-only (re-entry hangs), so durable resume uses
deterministic replay: suspend escapes to the driver; resume re-runs the flow and
replays resolved suspends from a (tag value) log. No live continuation is ever
serialized — persisted state is plain data, survives restart. Adds flow/start
(now state-returning, backward compatible), flow/resume, flow/cancel, store.sx.
Harness reuses one env with a per-test reset (full env rebuild 66x was too slow).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
