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>

lib/flow/conformance.sh

@@ -28,6 +28,7 @@ SUITES=(
   "distributed flow-dist-tests-run!      lib/flow/tests/distributed.sx"
   "api         flow-api-tests-run!       lib/flow/tests/api.sx"
   "combinators flow-cmb-tests-run!       lib/flow/tests/combinators.sx"
+  "railway     flow-rail-tests-run!      lib/flow/tests/railway.sx"
 )
 
 TMPFILE=$(mktemp); trap "rm -f $TMPFILE" EXIT