flow: Phase 3 suspend/resume/cancel via deterministic replay + 17 tests

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>

lib/flow/conformance.sh

@@ -23,6 +23,7 @@ VERBOSE="${1:-}"
 SUITES=(
   "basic       flow-basic-tests-run!     lib/flow/tests/basic.sx"
   "control     flow-ctl-tests-run!       lib/flow/tests/control.sx"
+  "suspend     flow-sus-tests-run!       lib/flow/tests/suspend.sx"
 )
 
 TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT
@@ -39,6 +40,7 @@ emit_eval () { echo "(epoch $EPOCH)"; echo "(eval \"$1\")"; EPOCH=$((EPOCH+1));
   emit_load "lib/scheme/eval.sx"
   emit_load "lib/scheme/runtime.sx"
   emit_load "lib/flow/spec.sx"
+  emit_load "lib/flow/store.sx"
   emit_load "lib/flow/api.sx"
   for SUITE in "${SUITES[@]}"; do
     read -r _NAME _RUNNER FILE <<< "$SUITE"