flow: railway attempt combinator — fail-value short-circuit + 10 tests (Phase 6)

(attempt n1 n2 ...) threads like sequence but stops at the first node returning a
(fail ...) value, returning that failure. Makes the fail/recover error model
compose into validation/ETL pipelines (railway-oriented). 132/132 across 8 suites.

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` → **122/122** (Phases 1-5 complete)
+`bash lib/flow/conformance.sh` → **132/132** (Phases 1-6 complete)
 
 ## Ground rules
 
@@ -149,6 +149,16 @@ something operators and authors actually use. Accumulation, not a rewrite.
   value while/until pred holds, capped at `max` steps (deterministic bound)
 - [x] `lib/flow/tests/api.sx` (12) + `lib/flow/tests/combinators.sx` (17)
 
+## Phase 6 — Railway-oriented composition
+
+Make the `(fail reason)` value channel compose into real validation/ETL pipelines.
+
+- [x] `attempt` — like `sequence`, but short-circuits at the first node that returns
+  a `(fail ...)` value, returning that failure (the railway track). Pairs with
+  `recover` for the rejoin.
+- [x] `lib/flow/tests/railway.sx` — 10 cases: fail short-circuiting, no-run-after-
+  failure, recover rejoin, validation pipeline reporting the failing stage
+
 ## Progress log
 
 - **Phase 1 (combinators + sequential runtime).** Flow built as a Scheme prelude