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>

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` → **36/36** (Phase 1 done; Phase 2 in progress)
+`bash lib/flow/conformance.sh` → **42/42** (Phase 1 done; Phase 2 in progress)
 
 ## Ground rules
 
@@ -77,7 +77,9 @@ lib/flow/spec.sx           lib/flow/runtime.sx          lib/flow/store.sx
 
 - [x] `cond` combinator — predicate selects branch (named `branch`; `cond` is a
   Scheme special form). `(branch pred then else)` — 6 tests.
-- [ ] `retry n [backoff]` — re-runs node up to n times on exception
+- [x] `retry n` — re-runs node up to n attempts on a raised exception; last
+  exception propagates. Only raised exceptions are retried — `(fail ...)` values
+  pass through. 6 tests. (Backoff deferred: no wall clock in pure SX.)
 - [ ] `timeout ms` — bounds node execution
 - [x] `try-catch` — exception handler with reified error: `(try-catch node handler)`
   runs node; on raise, calls `(handler error)` and returns its value. 6 tests.