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/tests/distributed.sx

@@ -86,4 +86,35 @@
     "(flow-peer-register! (quote p) (list (list (quote f) (lambda (x) (* x 2))))) (flow/start (sequence (remote-failover (list (quote down) (quote p)) (quote f) (flow-const 1)) (lambda (x) (+ x 3))) 5)")
   13)
 
+;; ── replication + handoff ───────────────────────────────────────
+(flow-dist-test
+  "replicate: a peer holds the exported store"
+  (flow-d
+    "(defflow w (lambda (x) (suspend (quote q)))) (flow/start w 10) (flow-replicate-to (quote peerB)) (if (flow-replica-get (quote peerB)) (quote replicated) (quote missing))")
+  "replicated")
+(flow-dist-test
+  "handoff: a peer resumes a flow after the local instance dies"
+  (flow-d
+    "(defflow w (sequence (lambda (x) (suspend (quote q))) (lambda (v) (list (quote done) v)))) (define id (car (cdr (flow/start w 10)))) (flow-replicate-to (quote peerB)) (set! flow-store (list)) (flow-restore-from (quote peerB)) (flow/resume id 55)")
+  (list "done" 55))
+(flow-dist-test
+  "handoff: restored peer reports the flow as resumable"
+  (flow-d
+    "(defflow w (lambda (x) (suspend (quote q)))) (define id (car (cdr (flow/start w 10)))) (flow-replicate-to (quote peerB)) (set! flow-store (list)) (flow-restore-from (quote peerB)) (flow-resumable-ids)")
+  (list 1))
+(flow-dist-test
+  "handoff: without restore the dead instance has lost the flow"
+  (flow-d
+    "(defflow w (lambda (x) (suspend (quote q)))) (define id (car (cdr (flow/start w 10)))) (flow-replicate-to (quote peerB)) (set! flow-store (list)) (flow/resume id 1)")
+  (list "flow-error" "no-such-flow"))
+(flow-dist-test
+  "restore: from an unknown peer yields false"
+  (flow-d "(flow-restore-from (quote nowhere))")
+  false)
+(flow-dist-test
+  "handoff: replication preserves the replay log across the move"
+  (flow-d
+    "(defflow two (sequence (lambda (x) (suspend (quote a))) (lambda (x) (suspend (quote b))) (lambda (x) (list x)))) (define id (car (cdr (flow/start two 0)))) (flow/resume id 11) (flow-replicate-to (quote peerB)) (set! flow-store (list)) (flow-restore-from (quote peerB)) (flow/resume id 22)")
+  (list 22))
+
 (define flow-dist-tests-run! (fn () {:total (+ flow-dist-pass flow-dist-fail) :passed flow-dist-pass :failed flow-dist-fail :fails flow-dist-fails}))