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/scoreboard.json

@@ -1,10 +1,11 @@
 {
-  "total": 49,
-  "passed": 49,
+  "total": 66,
+  "passed": 66,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
-    "control": { "passed": 31, "total": 31 }
+    "control": { "passed": 31, "total": 31 },
+    "suspend": { "passed": 17, "total": 17 }
   },
-  "phases": { "phase1": "done", "phase2": "done", "phase3": "pending" }
+  "phases": { "phase1": "done", "phase2": "done", "phase3": "in-progress" }
 }