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>

plans/flow-on-sx.md

@@ -16,7 +16,7 @@ federation extension via fed-sx for remote-node execution.
 
 ## Status (rolling)
 
-`bash lib/flow/conformance.sh` → **74/74** (Phases 1-3 done; Phase 4 fed-sx next)
+`bash lib/flow/conformance.sh` → **81/81** (Phases 1-3 done; Phase 4 in progress: remote-node done)
 
 ## Ground rules
 
@@ -116,7 +116,10 @@ lib/flow/spec.sx           lib/flow/runtime.sx          lib/flow/store.sx
 
 ## Phase 4 — Distributed nodes via fed-sx
 
-- [ ] `(remote-node addr fn args)` — execute node on a federation peer
+- [x] `(remote-node addr fn)` — execute a node on a federation peer. Transport is
+  the fed-sx boundary, MOCKED via a peer registry (`flow-peer-register!`); raises
+  `flow-remote-unreachable` / `flow-remote-no-fn`. Composes with sequence, suspend,
+  retry. `tests/distributed.sx`, 7 cases.
 - [ ] failure semantics — retry on different peer, fall through to local
 - [ ] persistence across instances — flow state replicates via fed-sx
 - [ ] handoff — flow started here can resume on a peer if the local instance is down