flow: remote-node via mock fed-sx transport + 7 tests (Phase 4 begins)

(remote-node addr fn) runs a node on a federation peer. Transport is the fed-sx
boundary, mocked by a peer registry (flow-peer-register!); raises
flow-remote-unreachable / flow-remote-no-fn. Composes with sequence/suspend/retry.
Also fixes conformance.sh to load remote.sx before api.sx.

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

lib/flow/scoreboard.md

@@ -1,6 +1,6 @@
 # flow-on-sx Scoreboard
 
-**All tests pass: 74 / 74 across 4 suites.**
+**All tests pass: 81 / 81 across 5 suites.**
 
 `bash lib/flow/conformance.sh`
 
@@ -12,6 +12,7 @@
 | control |  31  | Phase 2: `branch` (6); error model `fail`/`failed?`/`fail-reason` (6); `try-catch` (6); `retry n` (6); `timeout` cooperative step budget (7) |
 | suspend |  17  | Phase 3: suspend/resume/cancel via deterministic replay; multi-step, replay determinism, lifecycle guards, suspend-in-branch |
 | recovery |  8  | Phase 3: crash recovery — store export/import, resumable scan, restart-at-every-step, replay-log survival |
+| distributed | 7 | Phase 4: `remote-node` on mock fed-sx peers; compose/suspend/retry, unreachable + no-fn errors |
 
 ## Architecture
 
@@ -40,5 +41,6 @@ capture the flow continuation directly.
 - [x] Phase 1 — Declarative DAG + sequential execution (combinators + 18 tests, `flow/start`)
 - [x] Phase 2 — Control flow + error handling (branch, error model, try-catch, retry, timeout)
 - [x] Phase 3 — Suspend/resume (suspend/resume/cancel + crash recovery via deterministic replay)
+- [~] Phase 4 — Distributed nodes via fed-sx (remote-node done; failover + handoff next)
 - [ ] Phase 3 — Suspend / resume (the showcase)
 - [ ] Phase 4 — Distributed nodes via fed-sx