flow: retry combinator — re-run node on raised exceptions + 6 tests

(retry n node) re-runs up to n attempts on a raised exception; the last attempt's
exception propagates. Explicit (fail ...) values are NOT retried — they pass through.

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: 36 / 36 across 2 suites.**
+**All tests pass: 42 / 42 across 2 suites.**
 
 `bash lib/flow/conformance.sh`
 
@@ -9,7 +9,7 @@
 | Suite | Passing | Covers |
 |-------|--------:|--------|
 | basic |     18  | Phase 1: single nodes, linear sequence, data-flow threading, defflow, parallel fan/join, nested composition, publish-shaped flow |
-| control |  18  | Phase 2: `branch` (6); error model `fail`/`failed?`/`fail-reason` (6); `try-catch` reifying raised exceptions (6) |
+| control |  24  | Phase 2: `branch` (6); error model `fail`/`failed?`/`fail-reason` (6); `try-catch` (6); `retry n` re-running on raised exceptions (6) |
 
 ## Architecture
 
@@ -36,6 +36,6 @@ capture the flow continuation directly.
 ## Phases
 
 - [x] Phase 1 — Declarative DAG + sequential execution (combinators + 18 tests, `flow/start`)
-- [~] Phase 2 — Control flow + error handling (`branch`, error model, `try-catch` done)
+- [~] Phase 2 — Control flow + error handling (`branch`, error model, `try-catch`, `retry` done)
 - [ ] Phase 3 — Suspend / resume (the showcase)
 - [ ] Phase 4 — Distributed nodes via fed-sx