flow: replication + handoff across instances + 6 tests (Phase 4 complete)

flow-replicate-to copies the plain-data store export to a peer's replica slot;
flow-restore-from imports it. Handoff = replicate, local instance dies, peer
restores and resumes by id. The replay log survives the move, so all resolved
suspends carry over. Same durable-data mechanism as crash recovery, across
instances. All four phases complete: 93/93.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/flow/scoreboard.json

@@ -1,13 +1,13 @@
 {
-  "total": 87,
-  "passed": 87,
+  "total": 93,
+  "passed": 93,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
     "control": { "passed": 31, "total": 31 },
     "suspend": { "passed": 17, "total": 17 },
     "recovery": { "passed": 8, "total": 8 },
-    "distributed": { "passed": 13, "total": 13 }
+    "distributed": { "passed": 19, "total": 19 }
   },
-  "phases": { "phase1": "done", "phase2": "done", "phase3": "done", "phase4": "in-progress" }
+  "phases": { "phase1": "done", "phase2": "done", "phase3": "done", "phase4": "done" }
 }