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/scoreboard.json

@@ -1,6 +1,6 @@
 {
-  "total": 122,
-  "passed": 122,
+  "total": 132,
+  "passed": 132,
   "failed": 0,
   "suites": {
     "basic": { "passed": 18, "total": 18 },
@@ -9,7 +9,8 @@
     "recovery": { "passed": 8, "total": 8 },
     "distributed": { "passed": 19, "total": 19 },
     "api": { "passed": 12, "total": 12 },
-    "combinators": { "passed": 17, "total": 17 }
+    "combinators": { "passed": 17, "total": 17 },
+    "railway": { "passed": 10, "total": 10 }
   },
-  "phases": { "phase1": "done", "phase2": "done", "phase3": "done", "phase4": "done", "phase5": "done" }
+  "phases": { "phase1": "done", "phase2": "done", "phase3": "done", "phase4": "done", "phase5": "done", "phase6": "done" }
 }